The issue is, the pushback speed IS realistic compared to the realistic walking speed of the wingwalker. Just increasing the Pushback speed will either result in:
1) If we leave the wingwalker as it is now, he will be left behind
OR
2) If we increase his frame rate, he will walk at an unrealistic and comical pace
OR
3) We should redo the wingwalker animation to have him walking faster, but in a realistic way, but that will also means the speed will NOT be "customizable", because the new (faster) speed, which is fine for some users, will be the new default speed for everyone.
You really are expecting too much from FSX and its graphic engine: it's not that if I say to the wingwalker "set speed to 9 km/h" he will be so clever that he'll start to RUN...