the fix to all the frame drops was simply enabling multithreaded render submit
if your engineers can get that running without shadows popping/flickering it's easily a 50-60fps boost in cpu bound scenarios.
the other option is to allow us to completely disable the shadows/lighting in game and enable the option as a temporary fix, i went from 47fps in tangled shore to 112-140fps, this isn't a joke, the problem is the shadow flickering.
In most cases, shadows usually act weird due to shader problems. But as I don't know the specifics of the engine, that's the most basic estimate I can make. I agree that disabling shadows and Lighting should be a thing.
As for enabling multi-threaded rendering, I believe it's already being done. My suspicion lies with the changes introduced into the engine. The frame stutters could be side-effects of the changes.