Jesus… how many times.
1st - 90 drop to 45 in VR => oculus vsync, So it rly dont need to be a huge spike, just a little over 11ms. 2nd - try to use external profiler for gpu (if you think its gpu) like RenderDoc or intel GPA kit.
try to use Stat GPU or Stat rendering commands to see what exactly happening. Mb spike of drawcalls, or translucency ?