When working with a client who uses Customer Service we can use the Out of the box functionality to measure how long it took to do certain things with a case through SLAs. What we found out that you can not measure, is the total worktime from SLA expiration to Case resolved. Meaning that we can't in an accurate way see how many working hours it took until a case was actually resolved beyond the Case resolve SLA failing. If we set an SLA to complete when Case status = resolved the SLA will simply say "completed", there is no way to get the actual active time of the SLA. Doing a calculated field with datediff results in an inaccurate result due to it not considering the work schedule.

The idea is therefor: Stamp the total worktime of an SLA into a column on the KPI-instance entity OR display the timer when SLA is completed even after page refresh and stop the timer. That way you can see how many working hours you are overdue.

There is a similar idea but it does not really describe it the way I want to. If you MS consider these two the same idea, feel free to close this one.
https://experience.dynamics.com/ideas/idea/?ideaid=db0e959b-00b7-e611-80c2-00155d460f3c