T-Mobile now gives you even more data before throttling compared to other US carriers
Mind you, hitting the 30GB limit doesn't mean that T-Mobile will instantly lower your data speed. Rather, you will simply be more likely to get lower speeds during peak times and events of network congestion, because your connection is being deprioritized. Once normal activity resumes, or you enter a less-congested area, your connection will return to the expected level.
Currently, T-Mobile is the only carrier to have increased its throttling threshold and maintain a significant leverage in users' favor. Comparatively, Sprint stays at 23GB while Verizon and AT&T start throttling at 22GB.