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.