STEPS TO REPRODUCE: 1) Load http://treeherder.mozilla.org/ 2) Scroll down to the bottom. 3) Click the "50" button to load more results. 4) Open the web console. 5) Run this script: document.documentElement.style.display = "none"; document.body.offsetWidth; var start = new Date; document.documentElement.style.display = ""; getComputedStyle(document.documentElement).color; var mid = new Date; document.body.offsetWidth; console.log("Style: " + (mid - start) + "\nLayout: " + (new Date - mid)); We spend a lot of time on layout here. Under ProcessReflowCommands, 34% of the time is under nsHTMLScrollFrame::ReflowContents (for some flex item) which breaks down as 21% TryLayout and 13% ReflowScrolledFrame calls. Of course under TryLayout there is also ReflowScrolledFrame. Not really sure what's going on there... esp since the profile under ReflowScrolledFrame shows a bunch of time in GetStyleDisplay under ReflowBlockFrame, and some self time in nsBlockFrame::ReflowDirtyLines... plus some self time in nsHTMLReflowState::InitConstraints and the frame property deletions InitOffsets does. Seems a bit weird. :(

(I spun off bug 1159819 to cover flexbox inefficiencies, so this bug can be focused on scrollframe stuff. If it all turns out to be the same issue, we can dupe as-needed.)