I think I found the reason why people don't do this.

I was noticing that idkfa was running a little slow. I looked on the server, noticed there was other stuff going on, and thought nothing of it.

I noticed again this afternoon, and found that nothing else was going on. Yet idkfa was taking almost 2 seconds to load each time.

Looking closer, I found that the type of query I have to use to do this is not terribly efficient unless you index heavily, and index correctly. I had three indexes, one to keep track of the user ID of the message bound, one to keep track of the lower bound, and one to keep track of the higher bound. I removed these, and added a single index that contained a concatenation of all three rows (usr, lower, upper).

This improved speed by reducing load times back down below 200 ms.

I guess we'll just have to watch as this scales. If you folks notice more slowdowns, don't be afraid to mention it.