The following chart would be very helpful for identifying improvement "hotspots" along the workflow:

For a given time window (specified by the user), create a distribution chart showing the average time (Y axis = average time) spent on each workflow lane (X axis = lane name) by all relevant cards.

"Relevant cards" can be either only cards already archived that entered and left all/some workflow lanes during the chosen time window or all cards (archived or not) that entered and left all/some workflow lanes during the chosen time window.

If a card entered and left the same lane several times during the chosen time window (card was moved forwards and backwards many times), the total time spent on each lane should be used for the average calculation (instead of seeing each visit of the same lane as an individual card - this would wrongly reduce the calculated average time).