I don't use thunderbird but here it is said:

Disable Junk Mail Filter (IMPORTANT) - The adaptive Junk Mail filter in Thunderbird should be disabled for each account you set up. The Junk Mail filter thrashes whatever media it runs from (1,000 to 3,000 I/O writes just for marking a message as junk) which, for our purposes, will decrease flash ram life and cause the application to run *VERY* slow.