At the moment, if a workflow includes an approval step, then the time it spends waiting on_hold is included in the duration.
This makes it hard to do any analysis based on the time that the workflow is actually working. For example, the "trend" indicators displayed next to the duration are essentially useless because the on_hold time can be significantly different between one job and the next. It can also be significantly longer than the duration of the actual working time, meaning that increases in the working time are hard to spot.