I currently stay well over 125 nights each year across Marriott brands ----- and can tell you without any doubts that the internet service received by everyone in most of those hotels --- whether gratis to Golds and Plats or for a fee to others --- is the same for everyone in that hotel. Each hotel has only one internet provider serving all rooms and all guests --- the only difference is how the billing for Golds and Plats is handled. Now, if you speak of occasional slow speeds on a particular hotel system --- that's another issue and is usually caused by too many users on a system with too narrow a bandpass. When that happens, it is info you should provide to hotel management. FYI, management of a number of Marriott-branded hotels that I frequent tend to watch their internet speeds closely. And, some of them are negotiating with their providers for a wider bandpass (read it, higher internet speeds during periods of heavy use).
I don't think they have separate internet for paying and free. However, if you are in an older property that has the STSN terminal in the room, and they also have wireless, generally the wireless is faster. There are some properties I stayed in that have pathetically slow internet, and NOT only when there are lots in the hotel (Marriott New Orleans, who said they were getting more bandwidth this month, Marriott Houston Airport - who knows? dial up would have been faster). My point is that even if the managers "monitor" their internet, they are SLOW to get it up to speed. The pipe you had 10 years ago is not good enough today. You can't run 500 showers off a garden hose, and you can't run 500 hotel rooms over a T1.