Does flow separation starts earlier at higher airflow speeds?
It is generally assumed in incompressible aerodynamics that the Reynolds number over a wing is so high that the effect of the Reynolds number (same air viscosity, same wing chord, only variable in Re is speed) on the lift coefficient is negligibly small. I can think of two areas where this assumption breaks down:

Very small (for example RC model scale) wings, where the Reynolds number is appreciably reduced. As the air in this case is less turbulent, flow separation occurs earlier.
Compressibility effects such as shockwaves occuring on top of the wing at transonic speeds, where we have to include not Reynolds but Mach number in our lift coefficient estimation. The effect of Mach number on max. lift coefficient depends on the wing design, some fighters are optimized for it, while most aircraft have their max lift coefficient and max angle of attack reduced at transonic speeds.