Changes in camber are much more efficient close to the trailing adge, this is why the Cl does almost not increase with the deflection of a leading edge device.
You may also understand the deflection of a 10% LE flap as an deflection of a 90% TE flap and a rotation of the airfoil nose down (lower AoA) to bring the TE back to its position, in that case the effect of the 90% flap and the change in AoA cancel each other out more or less.
Despite the slats (slotted leading edge flaps) energizing the boundary layer by the high speed airflow through the slot and thus delaying separation, also drooping down the leading edge to the stagnation point reduces the pressure peak near the LE and the pressure increase gradient on the upper surface, also delaying separation. (you pressure distribution with droop nose down is less "peaky" and more smooth).
If the stagnation point moves below the physical leading edge at high AoA the airflow around the nose moves opposite to the direction of the general airflow producing very strong pressure peaks.