Layout talos regressions in Visual Studio 2015

According to https://bugzilla.mozilla.org/show_bug.cgi?id=1254767#c2 a
number of apparently layout related Talos figures regressed on PGO builds
performed with Visual Studio 2015. This is concerning and represents a
potential showstopper for switching automation to VS2015 this release cycle.

I'm sending this email to ask for help from layout technical experts to
help identify reasons for the regressions. Links to builds you can
test/profile with are in the bug.

I'm reasonably confident that the PGO profiling and optimization process
completed successfully. But don't rule out a failure there that's causing
layout code to not get PGO benefits.

Can someone please help us understand where layout code is slower with
these builds so we can switch to VS2015 in the next week or two?

gps