I've also been having issues with wifi connection to my VM super hub.
I think mine is the SH2, not the white one everyone complains about but the matt black one before it.
Anyway, since I updated iOS last week some time(11.4.1) I've had constant issues with slow connection on my iPhone 6S over wifi.
My MacBook, which I am currently using to post this, is fine and running on the same hub at the same frequency(5G setting rather than the 2G option), something I've done ever since getting my current hub, and had no issues with until recently.
I've tried getting help via Twitter but had no luck with their suggestions.
I've tried things like resetting my wifi network on my iPhone, which solved the issue for about 10 minutes.
Judging by the above posts, it seems like something that's wide spread and not getting fixed at the moment.