System starts to freeze when copying large amounts of data

OS is Debian 12. Got a brand new disk to use as a backup for a disk currently in use. For the initial rsync, my system starts to lag and freeze for maybe half a second at a time. This is especially noticeable when playing back audio. Load average jumps from ~5 to ~15 during the rsync. What is going on here? I know it's rsync because the instant I kill the process, audio playback goes back to normal. Trying to run with lowest cpu priority and lowest io priority like so: sudo nice -n 19 ionice -c 3 rsync -a --sparse --delete --info=progress2 /mnt/src/ /mnt/dest Running with sudo as a few files are owned by root and I want to be sure those get copied. How can I run rsync so my system doesn't freeze? Also I know it's not an issue with these specific disks, as I have tried other combinations of disks and I get the same issue. I also had this same problem on my older system, which used entirely different disks and motherboard.