You can use chart metrics of "Avg. 1st Response Time" and "Avg. Work Time". If you use "Status" as category, this will give you information on only closed tickets.
So simply open a new report, chose Ticket Reporting and in details define category as for example "Priority" and then chose chart metric of "Avg. 1st Response Time".
For a second report you can chose same Ticket Reporting and chose category "Status" and metric as "Avg. Work Time".
Then simply "Run" report.
Hope that helps,
I believe that what you are looking for is Chart Metric "Avg. Open Time" which computes for each ticket the difference between the time the ticket was open and the time it was closed (and averages the final values).
The "Avg Work Time" actually computes average time that was recorded on Tech Notes of the the tickets and doesn't reflect how long the tickets were open, only how much time the technicians actually spent resolving them.