Should I change my scheduler?

I just copied 4GB of data from one disk to another.

Whenever I do that, my machine becomes completely unresponsive until the data transfer task is done. I can't Alt+Tab to another application and do something else while the machine is copying.

I don't use any GUI applications for that. It's usually rsync, sometimes gcp.

I've been using Linux for 20 years and it hasn't always been that way. I used to be able to do other things while the machine was copying. Is there anything I can do to change this? Change my scheduler, maybe? I am currently using cfq.

Could you provide more information about the specs of this machine?

No solutions, but a couple of ideas for investigating:

Start top or htop, then start a data transfer and see where your memory is going.

Use journalctl -f to watch SystemD log entries in real time. See man journalctl for more.

what can we tell you about your system ?. Try it and see what happens.
Are the disks internal or USB connected ?.