System unresponsive during HDD activity

[Konstantin Svist]

  For some reason, while running HDD-intensive tasks (e.g. svn update on 
a large tree), system becomes extremely unresponsive.
And I don't mean tasks that require any disk access - I'm trying to type 
inside an ssh session in a terminal, and the remote machine is getting 
my keystrokes after a pretty long delay.

Just now, composing this email, thunderbird got stuck at the previous 
sentence for a second while it was downloading new messages.

What's going on??