Sometimes Mail and other programs leave a ton of clutter in the memory, slowing things down and causing trouble. Purge your memory and see what happens (No need to close any programs).
Open Terminal (spotlight search is easy way to get there), let the terminal load for a second, type "purge" and hit enter. The computer will seem to slow drastically for about 5 seconds or so, then all returns to normal.
This dumps all the stored memory that is not being used by programs. Sometimes there are memory "leaks" even after you quit a program. Purge the system and free up all that space. You'll be surprised how much faster things will run.