The 'Lazyload Images' filter is enabled by specifying
pagespeed EnableFilters lazyload_images;
Optimize browser rendering and reduce number of HTTP round-trips by deferring the loading of images which are not in the client's viewport. This filter implements the PageSpeed rules for optimizing browser rendering and minimizing round trip times.
lazyload_images filter defers loading of images until they
become visible in the client's viewport or the page's
event fires. This avoids blocking the download of other critical resources
necessary for rendering the above the fold section of the page.
The filter changes the
src attributes of
<img> elements on each HTML page to
pagespeed_lazy_src. It attaches an
handler to these elements to determine whether the element is in the
client's viewport, and if so, loads it. It also attaches an
onscroll handler to the page, so that elements below the fold
are loaded when they become visible in the client's viewport as the user
scrolls down the page.
Lazyload After Onload
Additionally, in 22.214.171.124 and later, all images on the page that haven't yet
been loaded will be forcefully loaded after the page's
is fired. This reduces jank when scrolling the page, especially on mobile
devices, at the cost of downloading bytes for images that may never be displayed
to the user.
To disable loading of images on page
pagespeed LazyloadImagesAfterOnload off;
By default, when the page is initially viewed, a 1x1 blank image is served from pagespeed_static. It is also possible to use the same image served by Google's network, or any image that you choose, by specifying:
pagespeed LazyloadImagesBlankUrl "http://www.gstatic.com/psa/static/1.gif";
An advantage of this approach is that this gif may already be in the browser cache since any PageSpeed-enabled site can share it. However, this would send traffic to Google with your page as the Referer.
These directives can be used in location-specific configuration sections.
Disabling Lazyloading Per-image
lazyload_images doesn't defer an
img tag if it has
<img pagespeed_no_defer="" src=.../>
To ensure images are only lazyloaded in supported browsers,
uses a User Agent blacklist. Browsers that will not see images loaded lazily
Lazily loading only images below the fold
As of 126.96.36.199, by default
a beacon to report back to the server the images that
are visible in the client's initial viewport (above the fold).
It takes a few accesses of a page for the data to be reported back and
processed but eventually the above-the-fold images will be known and will be
loaded immediately while all the other below-the-fold images will be lazily
loaded; until then all images are lazily loaded (and this is what the filter
did prior to 188.8.131.52).
The use of beacons can be disabled using the
directive. If they are disabled,
lazyload_images will not inject the
The effect of this filter can be observed by comparing the number of requests before and after rewriting. As you scroll down, you will notice that images below the fold are only requested after they become visible in the viewport.