One of my biggest complaints about Exchange 2007 has been how long it takes for EMS and EMC to start up. This especially becomes an issue with web-based front-ends for PowerShell scripts, which have an unfortunate tendency to time-out.
Now there is a tip on the PowerShell Team Blog which explains how to pre-compile the .NET frameworks using ngen.exe to pre-populate the Native Image Cache, thus speeding up the loading of all things PowerShell-based. The performance gains are impressive, but I suspect that the script would need to be re-run anytime Exchange or the .NET Framework get updated or patched.
I also note that there doesn't seem to be much of a speed boost on the Exchange servers, just on other boxes that use EMS or EMC to manage Exchange. On our web front-end for self-service mailbox management, the EMS scripts run about twice as fast!