I assume you mean that the instance handling the email request takes a long time to complete - rather than it blocking all instances of the weberver? The latter should never happen unless there is a shared mutex across all the PHP code (and even then it will only affect requests to PHP pages - not to static content).
Can you give an indication of how long its actually taking?
MTAs (mail transport agents) are all about queueing mails, and highly optimized to do so - So Frank Farmer's suggestion is a very bad one.
It'd be helpful if you gave us a clue about the OS and where the MTA is.
IME, long delays can be caused by bad DNS lookups - are you using ip addresses in your mail config or ip names? Have you got a smart relay configured for the local MTA (getting this wrong usually means that mail will get sent but only after quite a long pause).
Have you checked your MTA logs?
C.