Fixed host is a double-edged s-word...
It makes it so that everyone's connection to each other doesnt' matter, only the host, but it means that the host HAS to have a good connection to everyone in the room. It of course also means that if they leave or get disco'd, that the room dies.
IMO it's better most of the time to use non-fixed and have individual participants try to troubleshoot their individual problems...
I find that if I am racing with people I have never raced with before, (at least since new game was released) the game does not allow me to connect as well. NExt time I see them, I can actually SEE them on the track, where I couldn't before.
I think the network is learning, or being constantely adjusted to avoid specific problems. I say try out Fixed if you want to -- but be aware of its drawbacks -- and also wait and see how well they get this machine oiled up. It still has some work, to be sure.