This feature is enabled by default on Safari on the MAC. Sunspider performance gain I measured for QtWebKit is about 4 % (from 9987 ms to 9595).
Created attachment 43624 [details] proposed patch
Comment on attachment 43624 [details] proposed patch OK.
Comment on attachment 43624 [details] proposed patch Clearing flags on attachment: 43624 Committed r51288: <http://trac.webkit.org/changeset/51288>
All reviewed patches have been landed. Closing bug.
That's weird - SunSpider shouldn't regress from enabling threads, but it shouldn't progress either.