Adding a separate bug for this. I was hoping this would be fixed by <http://trac.webkit.org/changeset/107699> from bug 78590, but it appears to need more work.
We appear to be back to just a 2% regression since bug 78070 was filed. http://build.chromium.org/f/chromium/perf/xp-release-dual-core/moz/report.html?history=400&rev=122870
Oops, I read the graph wrong. Reopening and I'll comment on bug 78070.
The 2% regression has been made up for, so closing. 78070 remains open.