Been playing GT Sport over a 4G connection these last two weeks. It's been an eye opening experience and an almost universally horrible one.
The game will connect and I can browse lobbies and make full use of most the games features, however after I start and finish any activity I have to close and restart the game in order for it to reconnect, as it seems to time out.
The 'Save' feature has yet to record any mileage despite saying that it's saved the game every time I complete or do anything.
The game's connection will time out periodically and the only way to get it to reconnect is to close the app and relaunch it (this is unlike simply loosing a normal connection). This as you can imagine is tricky for league racing and/or lobbies. Often after an event has started I can either no longer connect to the lobby (and just sit at the lobby screen looking at everyone 'loading') or I'll have no liveries loaded (which is then often followed by a disconnect). Chatting in lobbies also doesn't work properly, no messages will work at all unless they are one of the few templates and include a player name.
Connection quality is also random. I joined a league race, completed a quali session without lag, then in the race my lag was so bad I was (according to everyone else) all over the place and then disconnected. However in another race and quali session (that combined lasted over an hour and a half) there wasn't a single connection issue.
When in the livery editor, in order to get decals to load you have to select and apply them within the first few minutes of loading into the editor, otherwise they simply won't ever load and the game will crash trying to load them.
I understand that GT Sport is an online game and that the offline content is massively handicapped, but even when being disconnected during an event or activity in GT Sport the game will simply tell you you've been disconnected and let you continue. However on a 4G connection you are left in a broken limbo where nothing properly works and nothing saves. You can just about get by but it looks a-lot less like a first party major racing game with FIA backing and more like a mod for AC.
It's kind of shocking just how poor the game operates over a 4G connection, the speed of my connection was good enough to play Starcraft 2 for example, and FPS's like CoD (with obvious lag spikes), but the fact the game barely functioned when operating menu's is another clear example of the messy and rushed state GT Sport still exists in. The content they are pushing out (for free) is commendable and impressive, but I wish they would just make the actual game stability and flexibility better.
I'm back on a fully fibre internet connection now, but I feel bad for the poor sods stuck with only 4G access wishing and trying to play one of the best current gen racing games.