basically what robotadam said (http://news.ycombinator.com/item?id=1579212). To clarify: the database lock is actually for the whole process (-> all databases). I had an especially bad experience with it before they introduced the yielding for long running operations. But even with the yielding, write heavy parallel things still suffer. Especially if your indexes ever get too big and the the system decides to swap in/out part of the index. I've seen 5-6 second inserts with hundreds of writes and reads piling up in the mean time (even without the swap part).

If only they had an equivalent of innodb as a storage engine, it would be awesome