Good Morning, Kent,
As I understand it, there are two major factors involved.
First, the shutter blades must open completely, so that they don't block light coming through the lens at the wide apertures. that means that the center area has light coming through longer than the edge area. The small iris opening at f16 or so tends to mean a little overexposure at higher speeds. At lower speeds, the opening and closing times are a relatively small percentage of the total open shutter time.
Second, a lot has to happen in a very short time with a short exposure: open shutter blades; come to a dead stop, close shutter blades. That's a lot to ask of a spring-driven device made with thin, delicate parts. No wonder many leaf shutters are a bit slow at higher speeds, especially as they age and any lubricant becomes either more ineffective or actually begins to impede the moving parts. In contrast, the focal plane shutter operates somewhat like a scanner at the high speeds; the travel speed doesn't change much, but the slit between the curtains is reduced. Even so, focal plane shutters are also a bit slow at high speeds.
If that doesn't all make sense, be patient. Someone else will probably explain it more clearly.