A 'benchmark' tag would be appropriate when discussing workout routines that focus on measuring performance. Doing a search on benchmark, 25 questions/answers are found (examples: [1, 2, 3, 4]) which is comparable to a lot of other tags that seem relevant. Subjects that might be relevant to tag with 'benchmark':
- Benchmark workouts in Crossfit,
- Performance benchmarks when running,
- Fitness tracking (devices, social networks).
One argument against this tag is that it may be too broad. An alternative could be 'benchmark-workout' as this is something one may want to discuss in particular.