We've been finding and fixing old, slow database queries left and right lately. We thought we were mostly done, but we just found another one. We rushed a temporary fix live (hence the proxy errors everybody got following the slowness) but we're working on a permanent fix now.
We won't stop until the site is reliable and fast all the time. We're making progress: normally we get all slow on Mondays, but yesterday was wonderful and we even hit an all-time bandwidth limit.
Watch lj_backend if you want more geeky talk.