I had just used tuxracer as an example for ease of reproduction. In reality I am experiencing the problem while trying to run several completely different OpenGL apps. Also, this is not a memory issue as the machine has 2GB of RAM... tuxracer isn't *that* memory hungry.
Today I discovered that if you switch to a virtual console and then back again while the apps are running (Ctrl-Alt-F1, then Ctrl-Alt-F7), it fixes the problem. All 4 copies of tuxracer will all run at speed. This definetly looks like a bug.