Performance of twitter login page is abysmal

[jdm]

Trying to type my username into the input box is a multi-second affair, with the display not updating for long periods of time. A profile trace shows 2.3 seconds for a "Painting" operation, with "PaintingPerTitle" operations taking >1s.

./mach run https://twitter.com/login

http://www.joshmatthews.net/twitter-login-trace.html

[metajack]

cc @pcwalton

With those kind of timings, this may be diagnosable with Instruments.app.

[jdm]

Instruments reports 16.7% under PaintContext::draw_box_shadow (for 6 separate threads). Of that, 15.8% under PaintContext::blur_if_necessary, which ends up spending 12.2% under mozilla::gfx::AlphaBoxBlur::Blur, which is entirely spent in mozilla::gfx::AlphaBoxBlur::BoxBlur_SSE2.

[jdm]

I can't test with webrender due to #11658.

[pcwalton]

I don't think it would be a good use of time to fix the non-WR path. Let's fix the WR crash.

[nox]

This doesn't seem to happen anymore.