I found a fix that worked for me in question: 4.5.1: Capped at 37.5 fps in VR - XR Development - Epic Developer Community Forums
Disabling SpeedStep in the Bios (CPU power saving technology) brought performance back to around pre-0.4.3 beta runtime for me (locked 75hz).
Running Win 7 Ultimate 64bit w/Aero off. Hardware: i7-4770K, 16GB Ram, GTX780 Ti.