In 2018, a study was done showing that MVNOs were ~23% slower with data speeds than going directly with any of the carriers. This is likely due to prioritization. The thing that never seems to be pointed out in articles like this is what happens when you're at a crowded event. Say you're at a car show or a baseball game . . . the local towers are congested, and, from what I understand, will prioritize traffic from direct customers (e.g., T-Mobile over Mint). Sometimes, even as a T-Mo customer, I am hurting for data speeds at events like this and people on Mint are likely getting unusably bad data speeds.
It's the one thing that keeps holding me back from switching to an MVNO. It also seems like the one thing that isn't going to change about it because the carriers want to keep their customers.
I would love to see an updated MVNO data speed study done . . .