I've seen this behavior in both sfx.p64 and #visitrack. I'm on a M1 Macbook Pro, OS X 14.3, running Picotron 0.1.0f.

1. Slow drags downward don't do anything - mouselock just returns 0 for dy, regardless of how far you drag. Slow drags upward do not have this problem.
2. Effective sensitivity is lower on Mac than on other platforms. I haven't verified this myself, but based on @drakmaniso's description of how the sliders should be acting in #visitrack, this seems to be true.

A few more problems related to mouselock:

1. It doesn't seem to work on the web player: mouse is not locked in place, and the mouselock function always returns 0 for dy.
2. On my computer (running linux, Picotron 0.0.1f), using a sensitivity of 1 (with mouselock(true, 1.0, 1.0)) makes the mouse cursor move 4 times faster than usual; I thought this was be because I have a 4k monitor and UI scaling of 200%, but changing these settings in the OS doesn't affect the result: only mouselock(true, 0.25, 0.25) keeps the same speed for the cursor.
3. There seem to be some kind of acceleration when the mouse is locked, which is not present otherwise in Picotron (and I don't have any mouse acceleration at the OS level).