It'd be a good expression to add, yeah. However in the meantime you can use a workaround by using two global variables to store the old x/y, and using a Distance expression to calculate the distance:
: Set 'Mouse Speed' to distance(MouseX, MouseY, global('OldX'), global('OldY'))
: Set 'OldX' to MouseX
: Set 'OldY' to MouseY
'Mouse Speed' will contain the distance the mouse moved since the last frame, which is effectively its speed.