As you know, we were on Regal for 14 days, commencing in Barcelona, ending in Athens.
2,300 pax on the first week, 2,800 on the second (approximately). Americans were in the majority, followed by Canadians, then Brits.
Only 3 sea days in the 14 days, so not much time for onboard "fun". The usual evening activities were available.
Personally, I had no issues with internet speed, but am only a sporadic user.
All those staying onboard for the Athens to Barcelona were tested on the last Friday of the cruise. I have no specific knowledge as to testing for those returning to the US/Canada, but anecdotally I believe testing was not offered