I had this problem before and it wasn't fixed. I guess I'm more looking for foresight over a possible solution at this point.
When I'm in a lobby of more than 10 (that's not the exact cut off, as I haven't tested it, but it's around there and 10 is always ok) my performance just dies. I get immense stuttering. My frames actually stay fine, but the game suddenly chugs as if I was only getting 4 or 5 fps. My car behaves strangely at that point and I have pretty much no control. People suggested adjusting the client total prediction time in the past, but that doesn't seem to do anything. I don't think it's performance related, though I do have an aging PC, because I can run Bots in large numbers completely fine. I have doubts that the issue is my internet, on a speed test just run I have 180 Mbps down and 12 Mbps up. So if you have any thoughts I'd love to hear them, DX diag will be attached.
But anyway, I know it'll be hard to answer, even if we knew what the issue was, but is this something I'm stuck dealing with, or is this a bi-product of being in development? I'm excluded from pretty much every multiplayer lobby because they end up getting too big for me to participate. In that instance I typically create my own server and limit the players to a number I know works but that's not a great solution because it doesn't always fill up, sometimes there's just not enough people on, or the more full lobbies draw them in more than mine does.