massive slowdown with 600+ requests open. limiting scan results to 100 - does not help.
not a massive ups hit, but it makes the game stutter (which makes stuttering belts, worse than the little ups down)
disabling all other mods does help, but it's still there (although barely noticeable)

Looks like both options does not have any effect at all. It's running 100% smooth and high ups when ghost scanner got removed.
If you need to, I could upload save file / videos taken.

after factorio restart, options seem to work again. but still, when it scans, belts stutter.

Going by your description your game is already at the point where it can hardly keep up.
Ghost Scanner will never be a lightweight mod, since my requests for api events for ghosts and logistics cell changes where denied.

Use smaller networks with fewer roboports and ghosts.

Same issue i faced with this mod. had to remove it sadly.