It does support 120Hz. There is an option in the settings for that.
The Lineage Recovery mentioned in this thread does not work in my case. It gets a blank screen after booting. And, I cannot downgrade the K30 5G speed edition to the required stock MIUI version where it gets fully bricked.
However, by using the suspicious closed-source unofficial "TWRP" published by some 3-rd party I found elsewhere (certainly a bad idea), this ROM works well after being flashed except that its screen is in also in blank for a short time. So I suppose it is not far away from fully working on the speed edition (although I do not how).