There is a problem in versions of AppleMail prior to version 1.1.2 of AppleMail. If the Fonts folder in your System Folder contains a large number of fonts earlier versions of AppleMail function slowly.
To increase the performance of AppleMail, you need to update the version of AppleMail to version 1.1.2. AppleMail 1.1.2 is part of System 7.5 Update 1.0.
This article can help you locate the software update mentioned here: