Just a note; of our current slowdowns, many are coming from locks on the 'old' table (which is still MyISAM, not the new table type). I'm going to try tweaking the sort buffer a bit, as I notice a lot of the stuck threads are in "sorting" state.
We may want to look at queries using the old table (reads: history list, diffs; sometimes show old version. writes: inserts on page edit; updates on rename; deletes on delete. There _oughtn't_ to be too much contention on this table.) and generally fix stuff up.
-- brion vibber (brion @ pobox.com)