Is lag caused by the game or a potato PC ?

Hello there,

Each time I'm playing in a lobby I created with my friends, the game is struggling in only 10-15 mins and it never gets better till the end of the game.

What is the reason of this lag ? I (host) have a very good PC/internet. My friends have average PC that can run it without problem in solo mode.

I'm so frustrated because that was the main problem in HD version... and now this version is going the same way...