Having the same issue, I think for whatever reason the wrong polling rate is being assigned to my trackpad. u can check ur trackpads frequency using evhz. 20 hz is laggy, 120+ is smooth.
Tried fixing it using modprobe but it says usbhid is in use.
so navigated to /etc/modprobe.d/
created new file mousepoll.config
added line options usbhid mousepoll=4
This sets my mousepoll to 250 after reboot instead of asking the device.
But this also only works half the time. sometimes it will randomly revert.
I'm using a thinkpad t480 and this was a problem on ubuntu too, so not an arch problem specifically.
also it wasn't like this last year, this is a recent problem in the last few months. at the same time my trackpoint and trackpoint buttons have stopped working and I've given up trying to make them work...
I think maybe the drivers for the trackpoint and trackpad are getting interchagably used. Dunno for sure.