Do all the great additions of Exchange 2013 SP1 have you thinking about turning on MAPI over HTTP? If so, are you running .NET Framework 4.5.1? The release notes for this framework mention that performance and reliability improvements are provided. New performance features such as ASP.NET app suspension and multi-core JIT improvements are now available; for instance, ASP.NET app suspension allows IIS 8.5 and Windows 2012 R2 to suspend sites based on request timeout values, thus freeing up CPU and memory on the server. When a suspended site is requested, the site is then loaded up into memory allowing for accelerated performance.
The Exchange Team echoes the performance improvements of .NET Framework 4.5.1 for 2013 SP1 customers in their latest blog post. Specifically, if MAPI over HTTP is going to be used within your messaging environment, then it is “critical” to install .NET Framework 4.5.1. The reason stated in this blog post is that the framework contains “important fixes that impact both performance and scalability of MAPI over HTTP at the CAS layer.”