I hope this is obvious but speed to perform a task is not the only criterion against which you would want to rate a practitioner. In many cases, speed may not even be a consideration.
It is also interesting the OP points out only the performance extremes: high, low.
Except for extremely small teams, for mid- to large-sized teams, that dichotomy does not exist.
I think there are three facets that need to be understood when rating your team: 1) the criteria to use; 2) the performance distribution; and 3) the dynamics of having varying strengths and weaknesses that, cumulatively, create a high performing team.
1: Too often we narrow the definition of high performance to one or two criteria while minimizing or ignoring other metrics, similar to this OP's question. Focusing too narrowly on one criterion at the expense of others will cause the label of high performance to be administered to an otherwise mediocre performer.
2: What does the distribution of performance really look like on any team in any domain? Is it normally distributed? Skewed? Most current thinking is that performance is severely positively skewed, where most of your team are mediocre and very few are hyper performers. So perhaps in reality, assuming your criteria are sound, you are really only identifying the hyper performer who might simply stand out.
3) If you only look at your team in a single dimension, you could easily lose sight of other strengths that exist with an individual who may appear mediocre against your criteria, but whose value contributes to the overall team's performance. A slow performer--looking only at speed as a criterion--may offer a high degree of precision in his/her work that could be exploited from a teaming perspective to enable quality in the team's process. So grooming that type of person in that particular team role will enhance the overall team's performance, which is far more important than the individual's performance itself. And remember the concept of The Apollo Effect.
In my practice, I don't bother looking at an individual per se. I look for key strengths to exploit to fill the team's requirements and help the team evolve holistically with those roles. Hyper performers are rare and will simply present themselves over time. Extreme and quite rare low performers will also present themselves and are naturally or unnaturally selected off the team over time. Overall, the most important metric to measure is the team's performance with the underlying assumption that your team will be filled with mediocre to average performers, statically speaking.