T-mobile internet

This has probably been asked a lot, but I can’t find it.

I'm toying with the idea of ditching my local ISP and moving to T-mobile home internet.  T-mobile is already our cell phone carrier.

Here is my concern.  I'm getting about 300mbps and voice lags video by ~half a second.  3-4 times an hour, when using fast forward, we will get the dreaded buffering? circle that can last a minute or 2.  Annoying.  I've seen posts here from people getting 70mbps and seem quite happy.  Is speed not that important?  Can we watch 3 TVs at a time, 1 tablet, and 2 cells and not experience lag?