Christine,
The Lead Performance Report doesn't let you group leads by lead owner, but there is a workaround:
1) Create a smart list for each lead owner with the criteria "Lead Owner Email Address is {Somebody@usp.at)"
2) In the report setup, add "Custom Columns"
3) in the popup, enter the name of each smart list.
4) these smartlists will now show a count in a unique column for each lead owner
I'd group the leads by month so you can see a trend of leads assigned to each owner over time.
-Steven