It's not really a straightforward answer. Most groups will have a set time at which the call guy(s) takes over, which is considered to be the end of the workday. If you're exceptionally fast, you're not going to be allowed to just disappear an hour or two earlier. There's almost always more work to be done, but you can find ways to take mental breaks that slower partners can't. If you're slow, then you may feel compelled to stay later to make sure that your production is within the accepted range for the group. If you don't meet production expectations, then the best case scenario is that you're already a partner who is simply resented by others. Worst case scenario is that you find yourself unemployed.
That's not to say it's all about studies read. Adding value in private practice comes in a lot of different flavors. Some people are RVU superstars. Others are willing or able to develop connections with hospitals and referring providers by sitting on committees, etc.. Still others are willing to do the work that few others are, like covering mammo. I think that once you work with someone for awhile, you develop a very good sense of just what they're worth, but only in a qualitative sense that isn't necessarily reflected in the cold, hard numbers.
