I found that Liferea (my rss reader) was starting to get really slow. Hitting space bar to go to the next article was getting somewhat painful levels of delay.
Liferea uses sqlite to store it’s information. Over time, when you delete from sqlite, the db gets pretty unoptimized (lots of tombstones). This can be fixed with the following command (make sure Liferea is NOT running before doing this):
sqlite3 ~/.liferea_1.4/liferea.db vacuum
(adjust the directory if running a different version of liferea.) It both made my liferea.db 1/2 as big, and now everything is snappy again.