I am excited to announce we have just added slow card tracking in Corrello. You can use this to track any cards in Trello which have got stuck in progress for longer than usual in any list.
We also show the average card age per list:
This is all very easy to configure. By default it shows cards as slow if they have been in the list longer than the 85 percentile line for recent cycle times. You can override that and choose you own time for each list though if you prefer.
This is great for finding cards which are stuck for some reason of for heading off cycle time issues before they happen.