Slow performance in Thunderbird

Slow performance when deleting e-mails in Thunderbird. 
 e.g. user will select a message in the inbox, hit the delete key, and deleting will take a measureable amount of time— several tenths of a second or longer. 
 User had several thousand e-mails, but they were all already sorted into multiple folders. 
 Solution: Go to File menu → Compact Folders. 
 Probable cause: User had received excessive amounts of spam or other e-mail that the user deleted over the last three months. Evidently even over such a short amount of time, enough e-mail was deleted to cause a performance hit. Compacting the folders solved this problem.