This has been my experience with the "budget" (MVNO) carriers as well... Mint, US Mobile, and even one of the cable company branded mobile services. Two devices, same model, same location, same signal strength -- MVNO will be unusable, while direct service from the carrier will have great throughput.Another factor to consider is that the budget carriers resell "unused" capacity of major carriers. For example, Mint with T-Mobile. This makes them cheaper, but if the major carrier they rely on is busy enough, your own data won't get through. It's not about whether you've used your own allocated data, but how busy is the carrier they rely upon at that place and time.
My adult daughter is pretty happy with Mint, but she does get slow or non-service sometimes despite having 5 bars of signal strength. The cost-reliability tradeoff works for her. That's a personal choice, but everyone should make it fully informed.
I live in an area where population growth is outpacing infrastructure growth -- so experience will absolutely vary. FWIW, I have AT&T (FirstNet) service and am very happy with it.