Why are Leading Edge Flaps (LEFs) create lift while down at slow speeds but decrease lift while down at higher speeds?
Leading edge faps also known as slats don't really create lift. The effect on lift coefficient is minor (less than 10% in general) but they help increase the stall angle of attack.
As a summary, flaps creates lift, by increasing surface area and profil camber whereas slats postpone the stall and allow for higher AOA by reducing pressure drop around the leading edge. This increases the theoretical maximal lift by increasing the stall AOA but doesn't change much the lift coefficient.
Slats are ment to work at high AOA by design, at higher speed your AOA is very small, the slat is thus not working properly, disrupting the flow on the suction side of the wind and reducing actual AOA, thus reducing lift of that particular wing while simultaneously increasing drag.