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
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