US CBP is severely understaffed and they like to ask everyone 50 questions, including citizens. When I got off Wonder in Miami, it was, I kid you not, a 95 minute customs line. Even worse, right before getting off the boat an elderly woman with trouble walking asked an officer how long the line is in the terminal and they told her 15 minutes. I thought "Oh good, should be pretty easy." Got off and into the terminal and I saw the longest line I've ever seen in my entire life. I'm not exaggerating about 95 minutes. I timed it.
Europe is better, but still a problem. It took 50 minutes when getting off Symphony in Barcelona. 20 minutes of that was due to a hold as customs stopped anyone else from getting off the ship because the terminal was too full.