Lowering the CPU priority isn't going to help you if one of the processes is disk-intensive, because disk reads and writes are uninterruptible by nature, which means the kernel cannot just make the other, high-priority process run in its place because disk access requires a CPU core as well.

Also, eventually, other programs will need to read data from the disk and if you have too many of those programs running, you run out of cores. And your system becomes unresponsive, your mouse freezes etc.

OP, how many cores does your system have?