First is an abridged lecture given by Frederick Handley Page describing the operation of what are, in fact, called automatic slots - the slats are simply the moving airfoil sections.
and, from 1939:
Operative words "the slot could be made to open at a pre-determined angle of incidence....make the wing stable at a large angles of incidence and so ensuring that, although the aircraft would stall, it would not "drop a wing" and go into a spin."
Therefore, automatic slots are not fully effective at all AOAs at low speeds, let alone combat speeds - which, BTW, have not been mentioned - until the wing has reached a certain, pre-set angle of incidence. What was the pre-determined angle of incidence for the 109? And at what speeds did the slots open?
Effectiveness v basic airfoil:
