You can check the report example in this example report in our demo account t with several formulas on how to calculate cycle times. They are a bit more complex and include some general optimization:
https://eazybi.com/accounts/1000/cubes/Issues/reports/499777-average-lead-and-cycle-time
You can check if there measure Average days in cycle from the report could work. It will count total time in a selected set of Transition statues (I used Transition status dimension with calculated member Custom cycle and status Category In Progress) to show total time spend in those statuses for resolved issues on resolution date. A filter by resolution date is the one providing better performance in those reports. The filter also ensures we pick up issues in completed cycles only.
If the formulas still work slow you would like to consider calculated JavaScript calculations for cycle times.
We have a feature request to support cycle time calculations by default in our backlog. I added the community vote to the ticket as well.
Daina / support@eazybi.com