Hmm. What graphics card do you have, for reference?
I don't *think* this is specifically a UI rendering issue; it seems more likely that it's more of a "turning off anything will help some" kind of situation, and the UI just happens to be a thing you can turn off. If you're getting 30 fps with vsync on, sometimes even a 10% performance boost can get you back to 60.
That's a bit counterintuitive, so to expand on that: to get 60 fps, the game has to do its calculations and render in 1/60th of a second, which translates roughly to 16 milliseconds per frame. If it takes even 17 milliseconds, and vsync is on - it's gone over-budget for the frame, and it has to wait the whole rest of the second frame before updating the screen. If this happens consistently, it drops the framerate by 30.
If you were to turn vsync off, however, then the framerate would be closer to the actual time taken per frame, but you'd get tearing. And a potentially inconsistent framerate, which, depending on personal preference, may feel worse than a consistent lower framerate. (The game would still try to cap it to 60 fps at most, but that can be turned off in settings.json.)
