[Webkit-unassigned] [Bug 208168] New: Remove render update throttling

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=208168

            Bug ID: 208168
           Summary: Remove render update throttling
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: nham at apple.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 391600

  --> https://bugs.webkit.org/attachment.cgi?id=391600&action=review

loading apple.com/mac PLT content with render throttling on/off

Currently, we disable render updates after the first paint for 500 ms while the page is actively loading. However, oftentimes our first paint heuristic selects a first paint that isn't particularly interesting (mostly background colors) and this paint throttler just makes the user look at a nearly empty page for 500 ms.

Antti tested removing this and found that it was a ~0.5% PLT regression on the Mac. This is because PLT only measures first paint, DOMContentLoaded, and page load done, and page load done regressed for some sites that do a lot of layouts and repaints while the page loads. However, we've committed a lot of PLT wins this cycle and I think we should spend some of that budget on removing render throttling since it's a better user experience to see a page consistently render new content while loading.

I've attached a screen recording of loading apple.com with render throttling on and off. On the left side, render throttling is enabled, and the user has to stare at a nearly empty first paint for 500 ms. On the right side, render throttling is disabled, and we render interesting content much quicker.

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