timb

How much RAM do you have installed in your machine? and how big of a file can you work on presently?
Celerons are not designed for extensive bitmap processing. A friend of mine did a personal comparison between his Celeron 500 with his G4 450. He ran the same complex mathematical routine through both. The G4 450 did it around 8 sec. while the Celeron 500 took around 80 sec ! It isn't designed to run fast so slow rendering is something you'll just have to deal with.
Another thing is that some brushes do tend to get bogged down even for me on my G4 and when working on 30+ MB files there is often a performance decrease.

stecyk66