The "3-second freeze when somebody enters the lobby" problem is definitely NOT a network or ISP issue, at least not directly. It is a problem with the game itself, or rather somewhere in the game save files.

TLDR: fully delete your game data files or use a different account for multiplayer.

Long Story:

I installed PCars2 a few weeks ago and have been playing it quite a lot with a Logitech G920 wheel (and occasionally experiencing the "gravel-road" bug, but that's another story). I played some career and some multiplayer, and although I noticed the occasional stutter (when somebody entered or left the lobby), it was quite stable.

Then a few days ago I started getting this 3-5 second freeze where the screen would stop dead, the wheel would keep whatever force it was applying, and the sound would be stuck as well. When it resolves, you're in the same exact place but everybody else is 3-5 seconds ahead! Lame.

One strange thing I noticed with this game is that every time somebody joins the lobby, I see my own name with the audio icon on the left side for just a second, as if I was talking on the headset. This happens every time sombeody connects, EVEN WHEN I HAVE NO MICROPHONE.

I read a lot of theories in this thread (and others), and a lot of people think it's due to a network issue, latency, bandwidth, ports, IPv6, etc. I have a mega-fast connection with XFinity, I added all the port forwards, all the tests, etc. Nothing worked. I'm a software engineer so I know a thing-or-two. Nothing worked.

This problem was reported on PC, PS4, and XBox, and across all kinds of network configurations. I'm on an Xbox, so I decided to try the game with my son's account (never played PCars2 on that account). Guess what: no stuttering. People were joining, leaving, and I could play online with not a problem! I ended up fully deleting my own account's game data and it worked there as well.

It's probably some kind of network or player cache file. Or possibly related to career progress, but that seems odd to me. Maybe it's some random data corruption... Anyway, I had progressed through three of the series in career mode, so I lost all that, but multiplayer is too fun so it was worth losing it. I think I'll use one accound for online only, and another just for career.

I hope this helps some of you, because I know this bug affects a lot of people (I've seen frozen cars in almost every practice/qualifying since). Aside from the numerous bugs, this game is awesome and I really hope they improve it in PCars3. Beef up the multiplayer and kill those bugs, please!