@inliquid esoTalk is something very fast even on calculator.
Like others have said, there's more to website performance than the http daemon serving it out. Personally I use Apache just because I already have it setup, and I don't serve out enough content to really hit limitations with Apache on my personal server. I've switched very high-load servers from Apache to Nginx and noticed a huge increase in performance.
@Niuwang I'd personally like Litespeed just because from my experience i'ts 533% faster than Apache and 67% faster than nginx..
Sorry to burst your bubble, but you just pulled those figures directly from this litespeed blog post talking about serving static content with keep-alives enabled. I would highly doubt esoTalk requests would be 533% faster on one web server vs another even with ideal conditions.
I think the subject that covers most performance issues is what your hosting solution is. If you're using free or very cheap shared hosting, you're competing with everyone else on that (more than likely) overloaded and oversubscribed web server. Beyond that, tuning esoTalk will be just like tuning any other php/mysql web application.