It looks like most of our time is spent in graphical operations to handle calls from JS into CanvasRenderingContext2D -- i.e. this is perhaps some <canvas> case that we haven't optimized but maybe should. Vicky compared against Chrome (on Mac), and says this site is much smoother there, so we have room for improvement.
Bug 1515473 Comment 3 Edit History
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In my webrender profile (comment 2), it looks like much of our time is spent in graphical operations to handle calls from JS into CanvasRenderingContext2D -- i.e. this is perhaps some <canvas> case that we haven't optimized but maybe should. Vicky compared against Chrome (on Mac), and says this site is much smoother there, so we have room for improvement.