Roomba Discovery 4120 barely moving

Question

My Roomba Discovery 4120 has suddenly slowed down to a crawl. I mostly disassembled it (couldn't get the white lid to full separate from the bottom piece) and pulled out lots of dust bunnies. We've got a cat that puts out a good bit of hair so I've always had a chore of cleaning the brushes and clearing dust out of the sensor areas.Any ideas on why it is moving so slowly? Thanks!