Is the definition of a good server in this case one who can serve more tables than others in the same amount of time? In most places tips are mandatory and % does not depend much on anything unless someone messed up.
Commission on sales is very different from restaurant tips.
The final cost of the bill matters more than anything. A server at a higher-end restaurant where the bills regularly exceed hundreds of dollars will earn more in tips serving fewer tables than a server that works at a cheaper casual-chain restaurant (IHOP, Applebees, etc).
More so than tables per unit time, it's dollars per unit time. When I was a server, the usual metric of how well you performed on a given shift was the total of your bills ("how much you sold"). The best servers were good at encouraging parties to spend on the things they were on the fence about: the appetizer, the second drink, the dessert. Even with the volatility of individual tipping decisions, getting your tables to order more increases the EV of your total tips.
Tips, at least in the US has never been mandatory but it is an immense social pressure. Besides, my understanding is that tipping % is arbitrary anyway - there have been studies that show good looking people get more tips.