Thank you both for the tips! I think I've figured out the problem, and I have a workaround, but not the best solution yet.
I've mounted this monitor sideways in an arcade cabinet, and I've set the Ubuntu displays control panel to rotate counterclockwise, so the Ubuntu desktop shows up properly - tall and narrow. But when I start a game in Mame with GLSL turned on, it's choppy. If I go into the Ubuntu displays control panel and set the rotation back to normal, then Mame runs nice and smoothly (though sideways).
So the workaround is to leave Ubuntu set to normal rotation (which makes the desktop sideways) and turn on rotation in Mame; this works fine and keeps the animation and sound smooth.
Since I'm not using this as a desktop computer, I guess I can live with a sideways desktop. :-) But it would be nice if I could get OpenGL acceleration working in Ubuntu with a rotated display, or if I could tell the hardware that the display is rotated.
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