You're going to run into two problems trying to idle or compress brain systems.
Evolution Has Already Favored a Lean System
Our brains use heuristic shortcuts, as well as a task manager which itself idles and brings into activity different subsystems. This is to minimize energy costs; there's a finite number of of thoughts you can manage on a handful of nuts and berries or a mouthful of animal flesh. It is for this cause that people are bad at the sort of "brute force" computing that machines do, though people often falsely attribute this to us just not being very smart. Beating nature tends to be more difficult than people anticipate.
But what about the subsystems we don't need because we're in a simulation?
The Brain Has a Habit of Sprawling Into Its Own Subsystems!
Close your eyes and picture the last thing you ate. If someone was doing a brain scan as you did that, your visual processing center just lit up (and maybe another system or two). Even profoundly blind and/or deaf people end up using the unoccupied sense-processing real estate, for other tasks. If you eliminated "unused" regions of the brain, there may be unanticipated consequences for basic function.
Did that one guy use his visual center to help him find the right word? Idling that subsystem just gave him aphasia. Plus, he can't perform visual abstraction tasks anymore. And that autistic lady who wasn't even good at recognizing faces (different, dedicated subsystem)? She is no longer able to quickly distinguish between images of different bacteria! And your blind mathematician ishas suddenly lost his mathematical intuition.
WorrseWorse than that, when systems fail, the brain rewires itself, so forcing subsystems to shut down will have permanent structural implications as the remaining structures change and adapt to losses, even if those losses were intended to be temporary.