So I have to say, I just had the weirdest issue ever with Skylines.. I played it with the new patch that adds European buildings and tunnels.. but it stuttered like nothing!
So I removed all mods and unsubbed all
It continued stuttering
I started with the cmd line arguments they list
It continued stuttering
I basically went insane there trying to find the cause, and you know what the problem was? Steam cloud! Steam cloud synced my modded savegames back and the game threw up whenever such a savegame was in the savefolder. If such a game was in the folder, game would stutter in a completely clean new game.
If that is not a wtf kind of issue, I don't know. Either way, it's worth nothing that "delete local contents" does NOT actually delete local game contents like.. uh, cloud synced saves in the save folder.. wtf steam!