OK, I found the issue. It was a driver bug. Switching to the video driver in squeeze-backports fixed things completely, and as a bonus redrawing in everything (not just in st) is much quicker now.
- [dev] st very slow to refresh when x is oriented left / righ... Nick
- Re: [dev] st very slow to refresh when x is oriented le... Mihail Zenkov
- Re: [dev] st very slow to refresh when x is oriente... Nick
- Re: [dev] st very slow to refresh when x is ori... Nick
- Re: [dev] st very slow to refresh when x is... Mihail Zenkov
- Re: [dev] st very slow to refresh when x is oriented le... Sam Watkins