How is nice/renice supposed to work? Am I misunderstanding?

I run two extremely CPU-heavy (but not memory or I/O heavy) task on my Macbook Pro with 12 cores. This made the computer unusable for other tasks. I therefore reniced these processes to 20 and set Firefox to -1 (all other processes are 0 or lower), but Firefox is still extremely laggy. Shouldn't Firefox get all CPU time it requires when its nice-value is the lowest among all running processes?