Yann I think you are wrong there, slats WILL decrease stall speed. The slot formed by a slat re-energises the boundary layer , delaying the separation point, allowing higher values of alpha and lift coeficient to be achieved before the wing stalls, therefore lowering the speed at which the stall occurrs. In addition, slats increase the wing area and camber. The lowering of the stall speed is one reason that on some Boeings, eg. 737, when the stall angle is approached the outboard leading edge slats extend automatically (inboard LEDs are flaps) delaying the stall. Too early retraction of leading edge slats can actually place the wing in a stalled condition unless speed is increased. Handley Page is generally credited with the design of leading edge slats way back in the 20s aor 30s. and he demonstrated some incredible angles of attack with some HP aircraft fitted with his slats.

Regards,

David Gladwin.