Wiz
As PBL says it is Mach number that reduces the AoA for separation of flow on top surface/stall/reduction in Cl max/buffet (or whatever characteristic you think of as marking the "stall")
At GA light aircraft speeds forget mach effects but with a high performance aircraft expect a reduction in the AoA at which stall effects are detected above about 0.3M. By the time you are at .8/.9 the max AoA you can sensibly use will probably be down by 2 or even 3 degrees depending on the aerofoil.
JF