Sluggish scheduling during a long disk copy

[Dan]

Hello. I am copying one 150G disk to a 3ware mirror, and the machine
becomes downright unusable. It takes seconds to switch between 'konsole'
windows and it takes seconds between I type a command (ssh session to
a remote box, which I know is fast and unloaded) and see it executed.

This is 7.1-BETA2. Is this a bug or a generally-accepted performance
behavior? 

Thanks!