I've tested this on another PC now and am getting the same results.
After doing some more testing with all of the available options, things are worse than I expected.
So to be clear, this is a 100Hz display, and until I tried the OpenGL renderer, the D3D renderer worked at 100 FPS.
When I start up Theater View, it is capped to 61 FPS now:
If I alt-tab away to another program and switch back, the cap drops to 50 FPS rather than 61. (1/2 the refresh rate)
If I go to General > Video Card > Hardware Accelerated Graphics and
disable V-Sync, Theater View is then capped to 33 FPS (1/3 the refresh rate) with
constant stuttering:
Again: The OpenGL renderer works at 100 FPS:
But it stutters very badly when scrolling lists, and will often just turn the screen all white or all black, requiring me to quit and re-open Theater View.
All of these options ignore the framerate setting, whether it is set higher or lower.