Why not T-Mobile?
Trade offs!
1. Better and more reliable coverage 2. Roaming surcharges for my (Google Fi) calls where less (albeit, nominally so) at $0.20/min vs $0.25/min 3. While she had some problems enabling her data, the transition of my service in this respect was absolutely seamless (and as just fast as here in the States) 4. Importantly, Google Fi supports tethering abroad whereas T-mobile does not
I suspect that their partners in europe throttle with an algorithm so aggressive it flat-out breaks TCP.
Remember when web pages were measured in kilobytes? Pepperidge Farms remembers.
The last time I tried using it overseas, text-heavy sites like HN loaded almost as fast as normal. Reddit was mostly usable, but certain subreddits were painfully slow due to image-heavy custom styles. But try to load an article from a typical news website, and the connection would time out before it managed to render anything.