That's a pretty big travel area, and T-Mobile's core network doesn't work well outside urban centers. (Rural coverage has historically been better on Verizon.)

T-Mobile's Extended Range network (LTE Band 12) has substantially improved their rural coverage lately, but is only supported on the US version of major cell phones. If your phone was purchased outside the US/Canada, it probably didn't have Band 12 support. There's a table here (the specific model number is important, in this case): http://www.spectrumgateway.com/compatible-phones

If you weren't picking up LTE, that also suggests to me you might have been having issues related to Band 12.

But outside that, near any mid-sized population centers (where Band 12 isn't needed), you should've had a fast signal. Especially in California. I just ran a speed test on my phone, running T-Mobile in suburban California, and got 90 Mbps down / 40 Mbps up. That's faster than my home internet connection.

I was on T-Mobile too with a EU phone. If you are using a EU phone, there is compatibility problems with the T-Mobile network. Unfortunately, I only learned about that from my friends after I bought your safe offer. The sales person didn't warn me in advance.