Lag (or latency) isn't PDs problem, it's the end-users. If you play Rocket League, it shows your latency or ping on the score screen.
Average latency is 40-80ms. At this rate the game runs smooth, you'll hit the ball when you should.
At 100-150ms latency, at the lower end you'll notice occasional stutters, at the higher end you'll find your car teleports all over the place.
At 150ms+ you'll have a lot of "lag" - the game becomes unplayable.
None of that is Pysonix issue as the connection strength and latency is dependent on the end users internet connection quality and their proximity to the servers.
GT7's player lobbies have the same issues, except they're all P2P, meaning if you create or host a player lobby, all players in the lobby act as servers. If any of the racers have a poor connection with high latency, everyone else will see that player as lagging and teleporting all over the place, to that racer themselves, everyone else will have lag issues.
Sorry, buddy but you don't fully understand what you're talking about.
Firstly, it IS PD's fault because of the server problems. In no way is it the end user that is at fault.
Video gaming only takes a small amount of bandwidth & if GT Sport worked fine, (which is the closest game to compare but still not a valid comparison), then it's down to the netcode.
Remember, GT7 was delayed, it was also a release disaster even with said release delay.
It took roughly 3-4yrs for GT Sport to become, essentially, playable without mishaps.
Stating another game to compare to it as you said, 'Rocket League', has absolutely nothing to do with GT7 & is in no way plausable as a comparison.
Secondly, a peer2peer connection does NOT make all players that join you, turn them into servers. That's the funniest thing I've ever heard in my entire life 🤣🤣🤣
The player opening a multiplayer lobby is the ONLY server that is created. Everyone else joins that server to be able to race.
Lastly, not aimed at yourself, users asking people to submit a 'speed test' will give you absolutely no information to the problem at all.
A speedtest is merely a snapshot of your connection at a given time, it WILL show you a comparison of what speed you pay for, compared to what you are able to achieve but it is impossible to diagnose a problem from it.
The ONLY way to diagnose a connection problem is to see where/how your ISP connects to a game & ping each node to pinpoint a problem.
You will also need to make a BQM,( a broadband quality metre), this will show you, live, what is happeneing to your connection & is probably the easiest method to finding a problem.
I don't know you or your experience with servers or ISP's & I'm not here to patronise you but in general it sounds as if people have given you the wrong information about a lot of things.
EDIT: Missed information & spelling mistakes...