There's a good reddit, i think NoContract, where you can go to learn more about MVNOs. There are several tiers of them in practice and they each have their own "catches" and "advantages". I used Cricket many years ago when they had a punishing speed cap. In the modern days some of these caps have been relaxed, but as you suspected, prioritization is the main way the actual carriers differentiate themselves from the MVNOs that sell access to the same towers. The worst MVNOs have terrible priority and in any well-populated area congestion makes them super slow almost all the time.

The thing is, this is highly variable -- and also geographically variable -- and some MVNOs can now offer similar priority as a mainstream plan. US Mobile is one, which I've been using for a couple years. Their neat advantage is that they will sell you a SIM (or e-sim) that rides on your choice of the big 3, and they'll also let you port between them without any other change to your account. They call this "Tele-Port". Some people will do that even just to go on a vacation to a state with different "best carrier", since there's nothing stopping you.