Bandwidth/data speed slowdown.

I feel you might be right about them "being a victim of their own success" if we're going to be kind. Hilares you had to switch to EE to post ... 😵‍💫☺️

From looking through many posts on here about coverage/speed etc I don't know obvs but I think it's likely that although Smarty promise not to slow or throttle speeds Three's "active management" at busy times definitely does and I think they may well prioritise their own direct customers over Smarty. Although how they'd do that I don't know. 

In short at present it looks to me like there's not enough to go round. Other networks have to cope with congestion and if you read comments here from users eg in London the comparisons with Smarty often don't look good.

I don't want to get reprimanded by some Smarty " guru" or "pioneer" for not being on message but even though it seems to work for some (or used to) I don't think it's consistent, reliable or good enough for everyone. Yet. Maybe once they've finished tinkering with masts re 3g n merging with Vodafone but I suspect Three will still prioritise their own customers re too many users not enough capacity if that's the right word. If I'm wrong about that be good to hear from someone who definitely knows otherwise.