Mvno using Verizon towers have a good chance of slow downs. I use Verizon and a prepaid using Verizon towers, so I can compare them side by side and I have. The worst speed difference was Verizon was 1400 mgbps while the prepaid kicked in at .52. I tested both same time and twice with similar results and there was no major event going on. There are times they run the same also, prepaid has never ran that fast though. I've had to switch to the Verizon phone hotspot for streaming music when the prepaid started buffering to much. I've also had one time I was kicked off a phone call and couldn't call out, kept getting all circuits are busy message, Verizon phone call went thru..I tried prepaid again while on the Verizon phone and still was getting that message.
Mvno are cheaper for a reason, there's difference. Chances are you might not notice slow downs, if you do it will always be inconvenient when it happens. I travel so having a solid connection at highest level is important. I tried the prepaid to test side by side because it I could save money for service why not, but there were to many incidents that I decided to keep Verizon my main line. Paying more for Verizon I expect Verizon customers to have priority over mvno users, my test at least proved to me that Verizon customers do indeed have priority.
I know you weren't looking at a Verizon mvno, but I just wanted to chime in that there are differences on Verizon and I'm guessing there will be on other carriers also, if there wasn't why wouldn't everyone use mvno.
Also as a side note, customer service is usually difficult at best, might not when have phone support. And remember, they already have your money, refunds and credits are next to impossible. If your payment doesn't go thru for any reason such as new credit card and you forget to update it, your service is shut off immediately and the one I use I've seen reports their service didn't get turned back on till next day when the payment process happens again and the new card gets charged. Again, this isn't all mvno, but another reason they are cheaper it's less support.
Good luck with your decision. Saving money now days is important, hence why I looked at it.