A recent change that has been introduced on Blink has improved significantly the CSS properties performance https://codereview.chromium.org/169453006. This optimization could probably be backported into WebKit.
Created attachment 225644 [details] Patch
Comment on attachment 225644 [details] Patch Sure, r=me.
Comment on attachment 225644 [details] Patch Clearing flags on attachment: 225644 Committed r164995: <http://trac.webkit.org/changeset/164995>
All reviewed patches have been landed. Closing bug.