Moving an app from w98 to w2k I noticed delays, a significant drop in performance and glitches
Are you talking otherwise identical hardware (especially video adapter make/model) as well?
If not it's really comparing apples and oranges.
OpenGL appears to be more hardware dependend than DirectX was, unfortunately... Since the majority of games uses DirectX, OpenGL hardware support seems to be more of an afterthought to some manufacturers. :-/