Are you reading or writing PNGs? Are they 8-bit or 16-bit? If you have multiprocessing on, have you tried it with multiprocessing off?
What can you tell us about your system and your workflow? Are you running the latest updates to CS4?
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events