I would not describe the perceived effect as laggy. I think jittery is more accurate.
Basically, if the mouse is moved from left to right in a straight line using the trackpad, the distance between each drawn mouse cursor is uneven, because the OS is not receiving enough sample points to accurately track its position.
The same experiment using a mouse with a 1000Hz polling rate results in a straight line of evenly distributed mouse cursor.
This is crucial for a smooth experience as dragging windows or files around also inherit this issue