Steps to reproduce: 1. Load the log in the URL field (or any other tinderbox full build log). 2. Start a selection, and while holding down the mouse button, scroll towards the bottom of the document quickly. In my Shark profile, 69.5% of the time was spent on PresShell::GetPrimaryFrameFor, and 25.8% of the time was spent on nsTextFrame::SetSelected. The first one boils down to nsCSSFrameConstructor::FindFrameWithContent (which takes 61.5% of the processor time) and the second one to nsAttrAndChildArray::IndexOfChild (which takes 16% of the processor time).
I would imagine bug 500882 would help with the getting of primary frames part of this.
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