Sorry, but I think that restricting this issue to mesh networks is a bit of a red herring: there are many of using these devices who have this issue with regular, non-mesh networks.
I have made numerous attempts to resolve this slow wireless issue on 3 different SB2 units, but nothing seems to help.
Well, I do think that the SB2 wifi module seems to showcase more problem than only in mesh networks as i tested my TP-Link router as well with it. The speed gets troublesome even with this router, at distances where my other notebook keeps a better speed. I am disappoited that either of us has problems and a host of others users maybe as well. I just pointed to the fact that mesh networks with their density advantage fare no better sadly with SB2.