I have a P4/2.4GHz - 1GB RAM machine and I find performance is good when I have "validate source when idle for..." set to the maximum setting. That way it'll only validate (perl -c) when I hit CTRL-s to save.
My laptop is a P4/1.8GHz - 512MB RAM, and again performance is good as long as the validation is set long.
I use Eclipse on my G4 mac at home (768MB RAM) and it's a slow dog. Painful.