- Are fastcgi processes actually the bottleneck?
- How many visitors are you expecting?
- How many are you expecting at a time?
- How many requests would each visitor typically generate?
- How much memory can you dedicate to running fastcgi processes?
- How long does each process take to run (on average)?
- Can you leave your users hanging round for a second or two?
- What possibilities exist for caching images once they're generated and serving them up statically?
Well, you get the idea. There's a lot you would need to know before anyone can give a helpful answer.