Conscious thought and perception occupy a pretty small amount of our brain power. If you could offload computational tasks to portions of the brain that wouldn't actually need to do anything if you were in the matrix, you could have a surplus.

The visual processing portion of our brains, for example. We have a blind spot over our optic nerve and we're colourblind at our periphery. Our eye hardware actually kinda sucks and we have this massive software layer running on dedicated brain hardware