If your problem is one of performance, specially if it's related to dynamic content, I think nginx is a very good option in terms of flexibility, lighthttpd performs better than Apache in some cases but if your requirements are specific you may need some configuration with ngnix and a high-performace server like FastCGI or Starman.

perhaps you don't know but this is part of a common problem for large implementations, it's usually known as the c10k problem, you may want searching that term to find more detailed information