It's almost certainly on PD's end, and given the fact that this has been happening since gt5, it seems PD ported more code than I would have guessed. Honestly, it is likely a server side issue as well, since it had persisted so long. I am hopeful that given the new platform and FIA partnership, this issue will get more visibility and thus a fix. But hope in one hand...
That said, if you think it's your connection, i implore you to consider two things, what other games have a similar issue when playing online? How many other players are experiencing the same issue? To my experience, I've never had issues online with another multiplayer game of a similar nature, and a lot of other people in the league are reporting the same issue. They likelyhood of it being your connection is pretty slim I am thinking.
On the bright side Ardamike, at least GTS isn't the only racing game, and PS4 the only platform to play on. Dropping big bucks on a rig for one game, i can see being dishearting, but there are luckily plenty of fish in the sea.