Depending on what you're doing, I wouldn't expect to see a lot of improvement over just using an HttpHandler. I'd start by just writing the HttpHandler and seeing how it performs. If you need it to be faster, try looking more closely at the things you're actually doing while processing the request and seeing what can be optimized. For example, if you're doing any logging to a database, try writing to a local database instead of across a network. If it's still not fast enough, then maybe look into writing something lower level. Until that point though, I'd stick with whatever's easiest for you to write.
For reference, I've written an ad server in ASP.NET (using HttpHandlers) that can serve an ad (including targeting and logging the impression to a local database) in 0-15ms under load. I thought I was doing quite a bit of processing - but that's a pretty good response time IMHO.
Update after several months:
If you clear all the HttpModules that are included by default, this will remove a fair amount of overhead. By default, the following HttpModules are included in every site via the machine-level web.config file:
- OutputCache
- Session (for session state)
- WindowsAuthentication
- FormsAuthentication
- PassportAuthentication
- RoleManager
- UrlAuthorization
- FileAuthorization
- AnonymousIdentification
- Profile
- ErrorHandler
- ServiceModel
Like I said above, my ad server doesn't use any of these, so I've just done this in that app's web.config:
<httpModules>
<clear />
</httpModules>
If you need some of those, but not all, you can remove the ones you don't need:
<httpModules>
<remove name="PassportAuthentication" />
<remove name="Session" />
</httpModules>
ASP.NET MVC Note: ASP.NET MVC requires the session state module unless you do something specific to workaround it. See this question for more information: http://stackoverflow.com/questions/884852/how-can-i-disable-session-state-in-asp-net-mvc
Update for IIS7: Unfortunately, things aren't quite as simple in IIS7. Here is how to clear HTTP Modules in IIS7