Lag of what precisely? The touchpad, the driver, the software or how it is rendered?

All of that together, the end-to-end latency between moving your finger and the result appearing on screen. The trackpad firmware & driver, the display compositor, the hardware cursor overlay (or lack thereof) combine to give you the total latency which makes using a non-mac touchpad frustrating.

You can observe this effect by comparing a magic trackpad 2 vs the internal macbook trackpad, when connected over bluetooth: there is a small but noticeable latency that makes the external trackpad less precise and less "fun" to use than the internal one.

Edit: you can observe the same difference between a standard corded mouse vs a bluetooth mouse. Or, in my case, a 125Hz USB mouse vs my PS2 mouse back in the Windows XP days - the difference was night and day.