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Inconsistent reporting for responsive images

I've just set up Page Speed Check and noticed there is inconsistency between testing for images when using <picture> as specified at

Some tests are reporting 331.17 KB, others report 182.60 KB; this is the difference between download of 3 images that are 320px square, and 3 images that are 640px square.

This could be for three reasons:
- A difference in "device width" at different render locations
- A difference in "pixel density" at different render locations
- A difference in interpretation of the <picture> tag (some tests may be using the fallback image)

Are there any plans to look at this issue?

Also for future it would be great to be able to specify/simulate pixel density and device width, so that load times can be recorded historically for both 1x and 2x devices. Mobile speed and size vs. desktop speed and size. At the moment it's basically muddling both.

p.s  A smaller screen doesn't necessarily correlate to the smaller image for my setup.

Scott Lawrence Answered

Official comment


Hello Scott!

Well the settings should be the same for each time, so in theory the image request size should not change from test to test, so there shouldn't be any inconsistency. It might have something to do with race conditions, and if you don't mind it would be great if you could open up a Support Ticket at so that we can look into this a bit more and figure out where the inconsistency is coming from! If you do open up a ticket it would be great if you could include a bit more information, with examples from the reports where you have seen this.

(So no, we haven't had any plans until now to look at this issue, but we'd be glad to if you raise a ticket!)

As for the feature request - we do have plans on including mobile Page Speed testing, though it isn't included on any roadmap yet. As for being able to actually specify the pixel density and device width I will forward this suggestion to the developers.

Whenever we have an update regarding this, if it is added to a roadmap etc, we will update this thread!

Amanda Borg

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