The data you likely want here is how much time each CL took in each of the CQ stages (waiting for CQ to start working on it, waiting for try jobs to run, waiting for tree to open, waiting for throttling in CQ, waiting for the actual commit, etc.). The raw data is available in chromium-cq-status, but its aggregation still needs to happen outside. I'll update
http://crbug.com/397168 with more detail.
There is no magic query to chromium-cq-status to get a list of the longest CLs, but again, the length of each CL can be derived from the logged data and presented by trooper-o-matic or a stand-alone python script. I think we can start with a stand-alone script and port it to trooper-o-matic. Details will be in the bug.
Sergey.