[Webkit-unassigned] [Bug 226764] Complex flex layout is very slow to load

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=226764

--- Comment #2 from Sergio Villar Senin <svillar at igalia.com> ---
(In reply to Paul Guz from comment #0)
> Our dashboards are built using 'splitter' components that use flex.  A
> splitter may be either in a horizontal (flex:row) or vertical (flex:column)
> orientation.  Splitters are nested, potentially deeply, to divide up the
> dashboard.
> 
> We have found complex dashboards extremely slow to load on iPads.  Testing
> on Safari on an iPad Air 2 (MGLW2ZP/A) running iPadOS 14.6, the following
> fiddle takes 35 seconds to load. http://bitly.ws/e45Y

It isn't definitely fast on desktop (seems to take 2-3 seconds) but far from the 35 seconds you report on iOS.

> Compare with the following fiddle, with flex removed: http://bitly.ws/e45Y
> 
> The same is observed in Chrome on the same iPad.

I believe chrome on iOS uses WebKit so it's normal that you see the same behaviour

> Desktop browsers and Android tablets of a similar specification do not
> exhibit this slow performance.
> 
> I do not have easy access to MacOS or Linux at this time so cannot compare
> with desktop WebKit, nor can I test on Technology Preview.
> 
> A similar bug (https://bugs.webkit.org/show_bug.cgi?id=222202) was recently
> fixed, and I believe that fix would have been shipped with iPadOS 14.5. 
> Similarly to this bug, we use % widths.  However, unlike the test attached
> to that bug, we nest both column and row layouts.

Unfortunately I cannot know whether that patch was included in that release, not sure if anyone from Apple can comment on that. If the patch is not in that release then it might be very well the culprit (note that we observed a 4000% improvement after the patch is some benchmarks)

I understand the best test case you have is http://bitly.ws/e45Y. Could you try to clean it up a bit (removing all unneeded information) so that we could check what is causing that slow performance?

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