Definitely EVE Online's MMO secret weapon is Python: https://www.eveonline.com/article/stackless-python-2.7 - as well as its numerous ecosystem utilities like Pyfa, Evernus, used by tens of thousands per day.
Once it gets to a few thousand concurrent users they have to drastically slow down the server tick rate. It's like playing a turn-based strategy game at that point.
*Disclaimer: I quit Eve awhile ago.