Magic Mouse 2 vertical tracking issue on iMac 27
researching internet a minute ago with the exact same problem in the same hardware - iMac 27 late 2015 + Magic Mouse 2
I found the answer that solved my problem - just blew the optics's and vertical tracking speed is ok again. I should use an air compressed can but used my lungs instead but away from the mouse to avoid humidity as much as possible.it worked for me.