When you select a lot of messages to delete, the page takes forever to reload. I suspect that each message is deleted in an individual query to the database, and that this is making things very slow. It should be possible to optimize this.
Full disclosure: when I say "when you select a lot of messages to delete", I mean "the one time I selected a lot of messages to delete". I've only done it once, so we may be looking at an anomaly. However, I can rule out the interference by the mid-tick backup, this took place at xx:28.
Sidenote, god how I hate the midtick backup. I get that it's important and I'm not suggesting you remove it, but.