the database has grown large.
anytime there is an update (new posts, replies, edits) - the database gets locked so no other process or person can make changes.
these locks are slower than the reads. when there are several locks queued up, it slows things down.
at peak times, the lock times have been getting slower and slower.
i'm planning some database maintenance this week.
things should be resolved when I move to the new server - but I don't know if that will happen within the next month or not.