We've tracked down a 1%-2% regression in page load performance to:
From code inspection it looks like this is going to synchronously check for a proxy being enabled on every page load (assuming page loads are at least 5s apart). Checking for a proxy being enabled is likely expensive; if we really need to do so this probably has to happen on a path that doesn't block page loads.
Rolled out r185320 in r185818.
EFL and GTK port builds have been broken since this revert.
Build should be fixed; we'll use bug #145542 to try to land this again.