OptDebug build of V8 running JS unit tests have been getting slower and slower with passing V8 versions

[rber...@601t.com]

We've noticed starting with V8 6.2 that the run time of our JS unit tests have jumped from 8 minutes to 14 minutes. And then worse still with 6.4. 

Is there anything we can do to speed up optdebug builds on Windows x64 and UWP(if that matters)? build settings or SetFlagsFromString flags? We've tried a few things out and nothing seems to help.

We've noticed when running the same test suites in a loop the 2nd/3rd time can spike up to 7x slower or there about and then by the 5th iteration the run times improves considerably.