Bug 156614 has change the throttle factor for GuardMalloc WK2 to 1/2. But that doesn't seem necessary for 8-core Macs, only 12-core ones time out with 3/4 factor.
Created attachment 277549 [details] proposed patch
Created attachment 277551 [details] proposed patch Accidentally sent with a change I used for testing.
Comment on attachment 277551 [details] proposed patch Clearing flags on attachment: 277551 Committed r200175: <http://trac.webkit.org/changeset/200175>
All reviewed patches have been landed. Closing bug.