Performance tuning has always been a pet topic for enterprise applications.

When it comes to tuning apps like Hotmail, there are multiple challenges involved.

Here is a writeup from the performance engineers on how they made hotmail more faster.

Though the article dwells on the familiar caching and other techniques, this is worth a read.

http://windowsteamblog.com/windows_live/b/windowslive/archive/2011/06/30/instant-email-how-we-made-hotmail-10x-faster.aspx