Deferredto speeds up Firefox by loading visible images first

Martin Brinkmann
Apr 15, 2013
Updated • Mar 28, 2019
Firefox, Firefox add-ons

Update: Deferredto is no longer available. A comparable extension that offers similar functionality is not available as well. End

Especially image heavy websites are often making use of a technique that is called lazy loading to speed up the time it takes to display the contents of a page on the screen.

The idea here is to load only those elements first that are needed to fill the visible part of the screen. Imagine opening a page with 100 images that are displayed in a column on the page. Instead of loading all or many of the images at the same time, only the images that you see when you open the page are loaded first, with the remaining images either being loaded when you scroll the page or after the loading of the visible content has ended.

Deferredto for Firefox uses a similar approach, only that it moves it from being implemented on individual websites - which you have no control over whatsoever - to the web browser so that it theoretically works on all pages you visit in the browser.

This works best for slower connections while high speed broadband users may not see a difference at all in their page loading times. The developer of the extension uses a placeholder system for images that have not been loaded yet using their width and height information so that the design of the website is not altered in any way by the extension.

If you are noticing slow downs, freezes or other issues while browsing image-heavy websites, you may want to check out the extension for the Firefox web browser to see if it improves your loading time of those pages in the browser. I'm on a 50 Mbit line and could not really notice any difference in regards to page loading speed (near instant is hard to improve).

The extension is based on the Deferredto script that the developer is making available on the official project website. Webmasters can use it to apply the same technology to their websites. Note that it requires you to load a third party script on your website.


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  1. Dmitry Kaigorodov said on August 27, 2013 at 2:23 am

    Hi, I’ve published the (completely) new (and still beta) version of Firefox DeferredTo plugin.

    Thank you Roger, I can (and will) fix this behaviour as in Firefox extension I can do more than just as a web page. Thank you.

    @Transcontinental I will run a lot of test soon. Now extension is completely different. Hope this will work for you too.


    The current implementation do not use any cloud service. Now plugin is
    Before I use cloud service but free service resource was quickly drained after I release the plugin.
    Cloud service have not stored anything to any database, just in-memory hashmap.
    I plan to develop paid version of this plugin with cloud service because hosting costs money.

  2. Anon said on April 16, 2013 at 4:38 am

    “deferred to plugin uses service to provide image width and height information to keep layout the same as if all images were fully loaded.”
    Is this addon secure? Will it improve anything if connection is slow because of jammed WIFI because of trying to connect an external server? Can anyone clarify if this addon is working locally or using some cloud service?
    In that case, how safe is this?

    1. Dmitry said on September 10, 2013 at 12:01 am

      Hi, Anon.

      Plugin does not use the web service now.

  3. Gordon said on April 15, 2013 at 6:53 pm

    Does anyone know something similar to this, but for Chrome. It sounds so useful!

    1. Dmitry said on September 10, 2013 at 12:00 am
    2. ilev said on April 16, 2013 at 12:40 am

      Chrome doesn’t need speed up add-ons (coupled with Adblock).

  4. Caspy7 said on April 15, 2013 at 3:27 pm

    This is *mostly* fixed in Firefox on the Aurora prerelease builds which will be released as final at the end of June as Firefox 22 (though you can download & start using Aurora now if you like).
    The current behavior will only decode & display images which are in view which improves both performance and memory usage.

    Unfortunately, on the initial page load, all of the images are decoded, but then those not in view are immediately discarded – making for an initial spike in memory (and a temporary, unnecessary hit on performance).
    This issue is on Mozilla’s radar and will hopefully be fixed soon.

    You can read the details here:

  5. Transcontinental said on April 15, 2013 at 12:09 pm

    I’ve tried it with an old computer and a 6MB bandwidth. Haven’t noticed any significant page rendering speed, but rather images displayed twice. First normally, then blank, then displayed again. Perhaps something to do with other settings, no idea. From what I read on several sites users seem enthusiastic so I might be missing something.

    1. Dmitry said on September 10, 2013 at 12:11 am

      Hi, I’ve rewritten plugin and now it must work much better.
      Now it stop image loading earlier. And also, the first images are not blocked at all. I believe the plugin is much clever now.
      Also it does the same with frames, iframes, flash and some other heavy resources.

      I would be thankful if you would give it the second chance and drop me a note.

      Thank you anyway.

  6. Sukhen K Mitra said on April 15, 2013 at 11:56 am

    Thanks a million, Martine

  7. Elben said on April 15, 2013 at 9:40 am

    It works. Thanks

  8. Roger said on April 15, 2013 at 12:12 pm

    “Lazyload” is a PITA! How To Geek has been using it for some time and I subscribe to their RSS feed. To save an article you must scroll to the bottom of the page to allow all images to load before you save. Many times I have opened a saved page later only to find that I don’t have any images. Having to go back and find the original page and re-download to get the images takes more time than I sometimes care to expend. In those cases I just dump the incomplete page that I have and move on.
    I fail to see where “lazyload” accomplishes anything but frustration! YMMV!

    1. Dmitry said on September 10, 2013 at 12:05 am

      Hi, Roger.

      Nice comment. I rewrite plugin and now Firefox will save the web pages without problems and you do not have to scroll down.

      Thank you for you help.

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