Lazy load

En algunos tutoriales incluimos la técnica de lazy load, para la cual usamos el plugin Lazysizes

Básicamente lo que hace es evitar cargar imágenes que no están dentro del campo visual en la pantalla del dispositivo donde se muestra el sitio. 

Cuando el usuario se encuentra cerca (en términos de scroll) de ver las imágenes, estas son cargadas. 

Para más información sobre esto, recomendamos que leer este artículo sobre cómo cargar imágenes para mejorar la velocidad de carga de tu diseño. 

Para cargarlo en tu diseño tenemos que seguir los siguientes pasos:

CSS

Agregamos los estilos dentro del archivo static/style-critical.tpl 

Si en tu diseño usas una hoja de estilos para el CSS crítico, vamos a necesitar agregar el código debajo dentro de la misma, pero si no es el caso entonces podés aplicarlo en donde tengas el CSS principal de tu diseño.

@keyframes placeholder-shine {
  0%{
    opacity: 0.1;
  }
  50% {
    opacity: 0.5;
  }
  100% {
    opacity: 0.1;
  }
}
.blur-up {
  position: absolute;
  top: 0;
  -webkit-filter: blur(4px);
  filter: blur(4px);
  -moz-filter: blur(4px);
  -ms-filter: blur(4px);
  -o-filter: blur(4px);
  transition: opacity .5s, -webkit-filter .5s;
}
.blur-up.lazyloaded {
  -webkit-filter: blur(0);
  filter: blur(0);
  -moz-filter: blur(0);
  -ms-filter: blur(0);
  -o-filter: blur(0);
}
.preloader-bg-img,
.product-slider-image.blur-up{
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  -webkit-filter: blur(4px);
  filter: blur(4px);
  -moz-filter: blur(4px);
  -ms-filter: blur(4px);
  -o-filter: blur(4px);
  transition: filter .2s, -webkit-filter .2s, opacity .2s;
  /* Avoid strange image behaviour on filters in IOS */
  -webkit-perspective: 1000;
  -webkit-backface-visibility: hidden;
}
.product-slider-image.blur-up{
  left: 50%;
  width: auto;
}
.swiper-lazy-loaded + .preloader-bg-img,
.lazyloaded + .blur-up{
  opacity: 0;
  -webkit-filter: blur(0);
  filter: blur(0);
  -moz-filter: blur(0);
  -ms-filter: blur(0);
  -o-filter: blur(0);
}



/* Used for images that have a placeholder before it loads. The image container should have a padding-bottom inline with the result of (image height/ image width) * 100 */
.img-absolute {
  position: absolute;
  left: 0;
  width: 100%;
  height: auto;
  vertical-align: middle;
  text-indent: -9999px;
  z-index: 1;
}

.img-absolute-centered{
  left: 50%;
  transform: translateX(-50%)!important;
  -webkit-transform: translateX(-50%)!important;
  -ms-transform: translateX(-50%)!important;
}

JS

Ahora vamos a necesitar agregar el plugin en el archivo external-no-dependencies.js.tpl o el archivo donde tengas los plugins de tu diseño.

!function(a,b){var c=b(a,a.document);a.lazySizes=c,"object"==typeof module&&module.exports&&(module.exports=c)}(window,function(a,b){"use strict";if(b.getElementsByClassName){var c,d,e=b.documentElement,f=a.Date,g=a.HTMLPictureElement,h="addEventListener",i="getAttribute",j=a[h],k=a.setTimeout,l=a.requestAnimationFrame||k,m=a.requestIdleCallback,n=/^picture$/i,o=["load","error","lazyincluded","_lazyloaded"],p={},q=Array.prototype.forEach,r=function(a,b){return p[b]||(p[b]=new RegExp("(\\s|^)"+b+"(\\s|$)")),p[b].test(a[i]("class")||"")&&p[b]},s=function(a,b){r(a,b)||a.setAttribute("class",(a[i]("class")||"").trim()+" "+b)},t=function(a,b){var c;(c=r(a,b))&&a.setAttribute("class",(a[i]("class")||"").replace(c," "))},u=function(a,b,c){var d=c?h:"removeEventListener";c&&u(a,b),o.forEach(function(c){a[d](c,b)})},v=function(a,d,e,f,g){var h=b.createEvent("CustomEvent");return e||(e={}),e.instance=c,h.initCustomEvent(d,!f,!g,e),a.dispatchEvent(h),h},w=function(b,c){var e;!g&&(e=a.picturefill||d.pf)?e({reevaluate:!0,elements:[b]}):c&&c.src&&(b.src=c.src)},x=function(a,b){return(getComputedStyle(a,null)||{})[b]},y=function(a,b,c){for(c=c||a.offsetWidth;c<d.minSize&&b&&!a._lazysizesWidth;)c=b.offsetWidth,b=b.parentNode;return c},z=function(){var a,c,d=[],e=[],f=d,g=function(){var b=f;for(f=d.length?e:d,a=!0,c=!1;b.length;)b.shift()();a=!1},h=function(d,e){a&&!e?d.apply(this,arguments):(f.push(d),c||(c=!0,(b.hidden?k:l)(g)))};return h._lsFlush=g,h}(),A=function(a,b){return b?function(){z(a)}:function(){var b=this,c=arguments;z(function(){a.apply(b,c)})}},B=function(a){var b,c=0,d=125,e=666,g=e,h=function(){b=!1,c=f.now(),a()},i=m?function(){m(h,{timeout:g}),g!==e&&(g=e)}:A(function(){k(h)},!0);return function(a){var e;(a=a===!0)&&(g=44),b||(b=!0,e=d-(f.now()-c),0>e&&(e=0),a||9>e&&m?i():k(i,e))}},C=function(a){var b,c,d=99,e=function(){b=null,a()},g=function(){var a=f.now()-c;d>a?k(g,d-a):(m||e)(e)};return function(){c=f.now(),b||(b=k(g,d))}},D=function(){var g,l,m,o,p,y,D,F,G,H,I,J,K,L,M=/^img$/i,N=/^iframe$/i,O="onscroll"in a&&!/glebot/.test(navigator.userAgent),P=0,Q=0,R=0,S=-1,T=function(a){R--,a&&a.target&&u(a.target,T),(!a||0>R||!a.target)&&(R=0)},U=function(a,c){var d,f=a,g="hidden"==x(b.body,"visibility")||"hidden"!=x(a,"visibility");for(F-=c,I+=c,G-=c,H+=c;g&&(f=f.offsetParent)&&f!=b.body&&f!=e;)g=(x(f,"opacity")||1)>0,g&&"visible"!=x(f,"overflow")&&(d=f.getBoundingClientRect(),g=H>d.left&&G<d.right&&I>d.top-1&&F<d.bottom+1);return g},V=function(){var a,f,h,j,k,m,n,p,q,r=c.elements;if((o=d.loadMode)&&8>R&&(a=r.length)){f=0,S++,null==K&&("expand"in d||(d.expand=e.clientHeight>500&&e.clientWidth>500?500:370),J=d.expand,K=J*d.expFactor),K>Q&&1>R&&S>2&&o>2&&!b.hidden?(Q=K,S=0):Q=o>1&&S>1&&6>R?J:P;for(;a>f;f++)if(r[f]&&!r[f]._lazyRace)if(O)if((p=r[f][i]("data-expand"))&&(m=1*p)||(m=Q),q!==m&&(y=innerWidth+m*L,D=innerHeight+m,n=-1*m,q=m),h=r[f].getBoundingClientRect(),(I=h.bottom)>=n&&(F=h.top)<=D&&(H=h.right)>=n*L&&(G=h.left)<=y&&(I||H||G||F)&&(d.loadHidden||"hidden"!=x(r[f],"visibility"))&&(l&&3>R&&!p&&(3>o||4>S)||U(r[f],m))){if(ba(r[f]),k=!0,R>9)break}else!k&&l&&!j&&4>R&&4>S&&o>2&&(g[0]||d.preloadAfterLoad)&&(g[0]||!p&&(I||H||G||F||"auto"!=r[f][i](d.sizesAttr)))&&(j=g[0]||r[f]);else ba(r[f]);j&&!k&&ba(j)}},W=B(V),X=function(a){s(a.target,d.loadedClass),t(a.target,d.loadingClass),u(a.target,Z),v(a.target,"lazyloaded")},Y=A(X),Z=function(a){Y({target:a.target})},$=function(a,b){try{a.contentWindow.location.replace(b)}catch(c){a.src=b}},_=function(a){var b,c=a[i](d.srcsetAttr);(b=d.customMedia[a[i]("data-media")||a[i]("media")])&&a.setAttribute("media",b),c&&a.setAttribute("srcset",c)},aa=A(function(a,b,c,e,f){var g,h,j,l,o,p;(o=v(a,"lazybeforeunveil",b)).defaultPrevented||(e&&(c?s(a,d.autosizesClass):a.setAttribute("sizes",e)),h=a[i](d.srcsetAttr),g=a[i](d.srcAttr),f&&(j=a.parentNode,l=j&&n.test(j.nodeName||"")),p=b.firesLoad||"src"in a&&(h||g||l),o={target:a},p&&(u(a,T,!0),clearTimeout(m),m=k(T,2500),s(a,d.loadingClass),u(a,Z,!0)),l&&q.call(j.getElementsByTagName("source"),_),h?a.setAttribute("srcset",h):g&&!l&&(N.test(a.nodeName)?$(a,g):a.src=g),f&&(h||l)&&w(a,{src:g})),a._lazyRace&&delete a._lazyRace,t(a,d.lazyClass),z(function(){(!p||a.complete&&a.naturalWidth>1)&&(p?T(o):R--,X(o))},!0)}),ba=function(a){var b,c=M.test(a.nodeName),e=c&&(a[i](d.sizesAttr)||a[i]("sizes")),f="auto"==e;(!f&&l||!c||!a[i]("src")&&!a.srcset||a.complete||r(a,d.errorClass))&&(b=v(a,"lazyunveilread").detail,f&&E.updateElem(a,!0,a.offsetWidth),a._lazyRace=!0,R++,aa(a,b,f,e,c))},ca=function(){if(!l){if(f.now()-p<999)return void k(ca,999);var a=C(function(){d.loadMode=3,W()});l=!0,d.loadMode=3,W(),j("scroll",function(){3==d.loadMode&&(d.loadMode=2),a()},!0)}};return{_:function(){p=f.now(),c.elements=b.getElementsByClassName(d.lazyClass),g=b.getElementsByClassName(d.lazyClass+" "+d.preloadClass),L=d.hFac,j("scroll",W,!0),j("resize",W,!0),a.MutationObserver?new MutationObserver(W).observe(e,{childList:!0,subtree:!0,attributes:!0}):(e[h]("DOMNodeInserted",W,!0),e[h]("DOMAttrModified",W,!0),setInterval(W,999)),j("hashchange",W,!0),["focus","mouseover","click","load","transitionend","animationend","webkitAnimationEnd"].forEach(function(a){b[h](a,W,!0)}),/d$|^c/.test(b.readyState)?ca():(j("load",ca),b[h]("DOMContentLoaded",W),k(ca,2e4)),c.elements.length?(V(),z._lsFlush()):W()},checkElems:W,unveil:ba}}(),E=function(){var a,c=A(function(a,b,c,d){var e,f,g;if(a._lazysizesWidth=d,d+="px",a.setAttribute("sizes",d),n.test(b.nodeName||""))for(e=b.getElementsByTagName("source"),f=0,g=e.length;g>f;f++)e[f].setAttribute("sizes",d);c.detail.dataAttr||w(a,c.detail)}),e=function(a,b,d){var e,f=a.parentNode;f&&(d=y(a,f,d),e=v(a,"lazybeforesizes",{width:d,dataAttr:!!b}),e.defaultPrevented||(d=e.detail.width,d&&d!==a._lazysizesWidth&&c(a,f,e,d)))},f=function(){var b,c=a.length;if(c)for(b=0;c>b;b++)e(a[b])},g=C(f);return{_:function(){a=b.getElementsByClassName(d.autosizesClass),j("resize",g)},checkElems:g,updateElem:e}}(),F=function(){F.i||(F.i=!0,E._(),D._())};return function(){var b,c={lazyClass:"lazyload",loadedClass:"lazyloaded",loadingClass:"lazyloading",preloadClass:"lazypreload",errorClass:"lazyerror",autosizesClass:"lazyautosizes",srcAttr:"data-src",srcsetAttr:"data-srcset",sizesAttr:"data-sizes",minSize:40,customMedia:{},init:!0,expFactor:1.5,hFac:.8,loadMode:2,loadHidden:!0};d=a.lazySizesConfig||a.lazysizesConfig||{};for(b in c)b in d||(d[b]=c[b]);a.lazySizesConfig=d,k(function(){d.init&&F()})}(),c={cfg:d,autoSizer:E,loader:D,init:F,uP:w,aC:s,rC:t,hC:r,fire:v,gW:y,rAF:z}}});


!function(a,b){var c=function(){b(a.lazySizes),a.removeEventListener("lazyunveilread",c,!0)};b=b.bind(null,a,a.document),"object"==typeof module&&module.exports?b(require("lazysizes")):a.lazySizes?c():a.addEventListener("lazyunveilread",c,!0)}(window,function(a,b,c){"use strict";function d(a,c){if(!g[a]){var d=b.createElement(c?"link":"script"),e=b.getElementsByTagName("script")[0];c?(d.rel="stylesheet",d.href=a):d.src=a,g[a]=!0,g[d.src||d.href]=!0,e.parentNode.insertBefore(d,e)}}var e,f,g={};b.addEventListener&&(f=/\(|\)|\s|'/,e=function(a,c){var d=b.createElement("img");d.onload=function(){d.onload=null,d.onerror=null,d=null,c()},d.onerror=d.onload,d.src=a,d&&d.complete&&d.onload&&d.onload()},addEventListener("lazybeforeunveil",function(a){if(a.detail.instance==c){var b,g,h,i;a.defaultPrevented||("none"==a.target.preload&&(a.target.preload="auto"),b=a.target.getAttribute("data-link"),b&&d(b,!0),b=a.target.getAttribute("data-script"),b&&d(b),b=a.target.getAttribute("data-require"),b&&(c.cfg.requireJs?c.cfg.requireJs([b]):d(b)),h=a.target.getAttribute("data-bg"),h&&(a.detail.firesLoad=!0,g=function(){a.target.style.backgroundImage="url("+(f.test(h)?JSON.stringify(h):h)+")",a.detail.firesLoad=!1,c.fire(a.target,"_lazyloaded",{},!0,!0)},e(h,g)),i=a.target.getAttribute("data-poster"),i&&(a.detail.firesLoad=!0,g=function(){a.target.poster=i,a.detail.firesLoad=!1,c.fire(a.target,"_lazyloaded",{},!0,!0)},e(i,g)))}},!1))});


!function(a,b){var c=function(){b(a.lazySizes),a.removeEventListener("lazyunveilread",c,!0)};b=b.bind(null,a,a.document),"object"==typeof module&&module.exports?b(require("lazysizes")):a.lazySizes?c():a.addEventListener("lazyunveilread",c,!0)}(window,function(a,b,c){"use strict";if(a.addEventListener){var d=/\s+/g,e=/\s*\|\s+|\s+\|\s*/g,f=/^(.+?)(?:\s+\[\s*(.+?)\s*\])?$/,g=/\(|\)|'/,h={contain:1,cover:1},i=function(a){var b=c.gW(a,a.parentNode);return(!a._lazysizesWidth||b>a._lazysizesWidth)&&(a._lazysizesWidth=b),a._lazysizesWidth},j=function(a){var b;return b=(getComputedStyle(a)||{getPropertyValue:function(){}}).getPropertyValue("background-size"),!h[b]&&h[a.style.backgroundSize]&&(b=a.style.backgroundSize),b},k=function(a,c,g){var h=b.createElement("picture"),i=c.getAttribute(lazySizesConfig.sizesAttr),j=c.getAttribute("data-ratio"),k=c.getAttribute("data-optimumx");c._lazybgset&&c._lazybgset.parentNode==c&&c.removeChild(c._lazybgset),Object.defineProperty(g,"_lazybgset",{value:c,writable:!0}),Object.defineProperty(c,"_lazybgset",{value:h,writable:!0}),a=a.replace(d," ").split(e),h.style.display="none",g.className=lazySizesConfig.lazyClass,1!=a.length||i||(i="auto"),a.forEach(function(a){var c=b.createElement("source");i&&"auto"!=i&&c.setAttribute("sizes",i),a.match(f)&&(c.setAttribute(lazySizesConfig.srcsetAttr,RegExp.$1),RegExp.$2&&c.setAttribute("media",lazySizesConfig.customMedia[RegExp.$2]||RegExp.$2)),h.appendChild(c)}),i&&(g.setAttribute(lazySizesConfig.sizesAttr,i),c.removeAttribute(lazySizesConfig.sizesAttr),c.removeAttribute("sizes")),k&&g.setAttribute("data-optimumx",k),j&&g.setAttribute("data-ratio",j),h.appendChild(g),c.appendChild(h)},l=function(a){if(a.target._lazybgset){var b=a.target,d=b._lazybgset,e=b.currentSrc||b.src;e&&(d.style.backgroundImage="url("+(g.test(e)?JSON.stringify(e):e)+")"),b._lazybgsetLoading&&(c.fire(d,"_lazyloaded",{},!1,!0),delete b._lazybgsetLoading)}};addEventListener("lazybeforeunveil",function(a){var d,e,f;!a.defaultPrevented&&(d=a.target.getAttribute("data-bgset"))&&(f=a.target,e=b.createElement("img"),e.alt="",e._lazybgsetLoading=!0,a.detail.firesLoad=!0,k(d,f,e),setTimeout(function(){c.loader.unveil(e),c.rAF(function(){c.fire(e,"_lazyloaded",{},!0,!0),e.complete&&l({target:e})})}))}),b.addEventListener("load",l,!0),a.addEventListener("lazybeforesizes",function(a){if(a.detail.instance==c&&a.target._lazybgset&&a.detail.dataAttr){var b=a.target._lazybgset,d=j(b);h[d]&&(a.target._lazysizesParentFit=d,c.rAF(function(){a.target.setAttribute("data-parent-fit",d),a.target._lazysizesParentFit&&delete a.target._lazysizesParentFit}))}},!0),b.documentElement.addEventListener("lazybeforesizes",function(a){!a.defaultPrevented&&a.target._lazybgset&&a.detail.instance==c&&(a.detail.width=i(a.target._lazybgset))})}});


!function(a,b){var c=function(){b(a.lazySizes),a.removeEventListener("lazyunveilread",c,!0)};b=b.bind(null,a,a.document),"object"==typeof module&&module.exports?b(require("lazysizes"),require("../fix-ios-sizes/fix-ios-sizes")):a.lazySizes?c():a.addEventListener("lazyunveilread",c,!0)}(window,function(a,b,c){"use strict";var d,e=c&&c.cfg||a.lazySizesConfig,f=b.createElement("img"),g="sizes"in f&&"srcset"in f,h=/\s+\d+h/g,i=function(){var a=/\s+(\d+)(w|h)\s+(\d+)(w|h)/,c=Array.prototype.forEach;return function(){var d=b.createElement("img"),e=function(b){var c,d,e=b.getAttribute(lazySizesConfig.srcsetAttr);e&&((d=e.match(a))&&(c="w"==d[2]?d[1]/d[3]:d[3]/d[1])&&b.setAttribute("data-aspectratio",c),b.setAttribute(lazySizesConfig.srcsetAttr,e.replace(h,"")))},f=function(a){var b=a.target.parentNode;b&&"PICTURE"==b.nodeName&&c.call(b.getElementsByTagName("source"),e),e(a.target)},g=function(){d.currentSrc&&b.removeEventListener("lazybeforeunveil",f)};b.addEventListener("lazybeforeunveil",f),d.onload=g,d.onerror=g,d.srcset="data:,a 1w 1h",d.complete&&g()}}();if(e||(e={},a.lazySizesConfig=e),e.supportsType||(e.supportsType=function(a){return!a}),!a.picturefill&&!e.pf){if(a.HTMLPictureElement&&g)return b.msElementsFromPoint&&i(navigator.userAgent.match(/Edge\/(\d+)/)),void(e.pf=function(){});e.pf=function(b){var c,e;if(!a.picturefill)for(c=0,e=b.elements.length;c<e;c++)d(b.elements[c])},d=function(){var f=function(a,b){return a.w-b.w},i=/^\s*\d+\.*\d*px\s*$/,j=function(a){var b,c,d=a.length,e=a[d-1],f=0;for(f;f<d;f++)if(e=a[f],e.d=e.w/a.w,e.d>=a.d){!e.cached&&(b=a[f-1])&&b.d>a.d-.13*Math.pow(a.d,2.2)&&(c=Math.pow(b.d-.6,1.6),b.cached&&(b.d+=.15*c),b.d+(e.d-a.d)*c>a.d&&(e=b));break}return e},k=function(){var a,b=/(([^,\s].[^\s]+)\s+(\d+)w)/g,c=/\s/,d=function(b,c,d,e){a.push({c:c,u:d,w:1*e})};return function(e){return a=[],e=e.trim(),e.replace(h,"").replace(b,d),a.length||!e||c.test(e)||a.push({c:e,u:e,w:99}),a}}(),l=function(){l.init||(l.init=!0,addEventListener("resize",function(){var a,c=b.getElementsByClassName("lazymatchmedia"),e=function(){var a,b;for(a=0,b=c.length;a<b;a++)d(c[a])};return function(){clearTimeout(a),a=setTimeout(e,66)}}()))},m=function(b,d){var f,g=b.getAttribute("srcset")||b.getAttribute(e.srcsetAttr);!g&&d&&(g=b._lazypolyfill?b._lazypolyfill._set:b.getAttribute(e.srcAttr)||b.getAttribute("src")),b._lazypolyfill&&b._lazypolyfill._set==g||(f=k(g||""),d&&b.parentNode&&(f.isPicture="PICTURE"==b.parentNode.nodeName.toUpperCase(),f.isPicture&&a.matchMedia&&(c.aC(b,"lazymatchmedia"),l())),f._set=g,Object.defineProperty(b,"_lazypolyfill",{value:f,writable:!0}))},n=function(b){var d=a.devicePixelRatio||1,e=c.getX&&c.getX(b);return Math.min(e||d,2.5,d)},o=function(b){return a.matchMedia?(o=function(a){return!a||(matchMedia(a)||{}).matches})(b):!b},p=function(a){var b,d,g,h,k,l,p;if(h=a,m(h,!0),k=h._lazypolyfill,k.isPicture)for(d=0,b=a.parentNode.getElementsByTagName("source"),g=b.length;d<g;d++)if(e.supportsType(b[d].getAttribute("type"),a)&&o(b[d].getAttribute("media"))){h=b[d],m(h),k=h._lazypolyfill;break}return k.length>1?(p=h.getAttribute("sizes")||"",p=i.test(p)&&parseInt(p,10)||c.gW(a,a.parentNode),k.d=n(a),!k.src||!k.w||k.w<p?(k.w=p,l=j(k.sort(f)),k.src=l):l=k.src):l=k[0],l},q=function(a){if(!g||!a.parentNode||"PICTURE"==a.parentNode.nodeName.toUpperCase()){var b=p(a);b&&b.u&&a._lazypolyfill.cur!=b.u&&(a._lazypolyfill.cur=b.u,b.cached=!0,a.setAttribute(e.srcAttr,b.u),a.setAttribute("src",b.u))}};return q.parse=k,q}(),e.loadedClass&&e.loadingClass&&function(){var a=[];['img[sizes$="px"][srcset].',"picture > img:not([srcset])."].forEach(function(b){a.push(b+e.loadedClass),a.push(b+e.loadingClass)}),e.pf({elements:b.querySelectorAll(a.join(", "))})}()}});

Listo, ya tenés el plugin listo para usar y podés seguir con el tutorial que estabas haciendo.