I've spent excessive amounts of time entering the UK border, including 45 minutes for "Fast" track once. CDG only had two agents managing the non-priority queue (exiting Schengen) last month when I was flying through as well which could seriously affect connecting flights. At least the USA doesn't bother with departure immigration when you fly home.
I don't think the airline can do squat about it except increase MCTwhich doesn't fix it, but alleviates stress over making it to your onward flight.