No idea. Invisible elements don't render themselves at all, they just return right at the beginning of their render function. But you can try commenting out the render loop here:

to see if it's element rendering. If that has no effect, try disabling individual mp.observe_property calls in main.lua to see which one is expensive.

I noticed insane latency (cursor line trailing way behind my actual cursor when hovering over the timeline) after updating. I bisected to mpv-player/mpv@851df70, turns out it's because of --gpu-context=vulkan. Switching back to the old default of --gpu-context=x11egl fixed my issue. May be the same thing here.