The secondary database server we use for +fav recommendation queries stopped responding for a while (possibly processing backed-up writes, it's only got hard disks, albeit six of them). These queries fell back to the primary server, but it wasn't able to cope with them at peak load and do everything else - which is why we put those on a secondary server in the first place.
Processing uploads deliberately has a low priority compared to everything else, on the theory that they tend to cause high CPU usage and high database traffic, so they were slowed the most.
We shifted the database queries back to the secondary and it's working now. Sorry for the inconvenience!
The secondary database server we use for +fav recommendation queries stopped responding for a while
