(This applies to the T-Mobile One plans, not other T-Mobile plans (such as pre-paid).)
I use T-Mobile cell service. I went to tether it to my laptop to get internet access on my laptop via tethering. Tethering uses your cell phones mobile data connection to create a wi-fi network that your laptop can connect to! Very handy! All providers offer this service. if you don't know how to do it, ask someone for help or google it.
Anywho, I noticed my tethered connection was running very slow (.5 Mbps download). So I checked to see why it was so slow. Turns out their new T-Mobile One plan limits your tethering speed to that by default, but you can increase it to full speed for free!
Logon to your t-Mobile account and then go here (Manage Data Add Ons). Then select this option:
It's currently free! I believe it is a promotion. Don't know when it ends, but do it now! Also, if you go to your account profile, there is an option there to get a kickback of $10 if you use less than 2 GB in any month.
Enjoy!