It appears that 1&1 is back to normal. Everything seems reasonably responsive, both on the front-end and in the admin. There’s been no explanation posted to the Twitter feed or on their status page, so I don’t really know what went wrong, or if it’s really fixed. That’s a little frustrating.
I started looking into the possibility of switching hosts today. Depending on how well 1&1 holds up, I may consider that. I’m paid up through November, so maybe I’ll revisit things in the fall.