1. Averages hide the worst waits
If nine guests get their car in 3 minutes and one waits 30, the average is 5.7 minutes, which looks fine. The guest who waited 30 minutes will not agree. Track your slowest requests (for example the 90th percentile) as well as the average.
2. Arrivals and departures by hour
Count arrivals and requests by hour to see when your peaks really are, and plan shifts so the full team is present before the departure rush.
3. Per-driver workload
Look at how many cars each driver handles and how long their retrievals take. Use it to balance work and coach, not to punish people for slow lots they could not control.