I'm experiencing a super weird behavior while pressing the selected modifier key and moving the mouse cursor.
When moving the mouse UP or to the RIGHT, the window follows at HALF the speed of the mouse cursor.
When moving the mouse DOWN or to the LEFT, the window follows at DOUBLE the speed of the mouse.
I'm using an iMac with a wired Logitech G600. A similar (though less pronounced) behavior can be seen when using the wireless Magic Trackpad (possibly due to fewer samples per second).