Making sure something happens exactly on time (despite GPU lag)?)

I’m making an application that needs frequencies to flash at exact frequencies because we’re analyzing brain signals so frequency precision needs to be exact.

My fps fluctuates because frames seem to be waiting on the GPU to process things sometimes. Is there a way to prioritize the rendering and logic of a certain object? (I.e. Ensure that the object updates at 90Hz no matter what and if the GPU didn’t find time to update a certain shadow, so be it)?