If TCP says "you can't send more data yet because of congestion control" (like hitting the congestion window limit) then I guess you can categorise that as late

If TCP says "you can't send more data yet because of congestion control" (like hitting the congestion window limit) then I guess you can categorise that as latency, depends on your viewpoint. RTT of acknowledgements shouldn't normally enter into it, no? You can send while previously unacknowledged data is in flight (as long as congestion control state allows).