Players who wait until the last second to make their first move

Is it that fun?

IDK

I don't do that tho

It seems rude

Some human minds work in mysterious ways.

I guess they are researching their opponent's games in those 15 seconds.

I used to think it was just a form of trolling, but now I use the grace period before the game to greet my opponent and to check their completion rate and block them if it's low, because I can forget after the game.

Sometimes it's just their internet connection.

Maybe they're trying to psych you out. Seems like it's working.

some times im doing something in a different tab and then i check the tourny only too realize that i have 3 seconds to make a move :/
luckily this doesn't happen to often

Yes.

I sometimes use that time to think what kind of game I want to play, if that makes sense. I dont really take until the VERY last second but idk its just my 2 cents

Personally, I don't think it's that big of a deal.
They're probably checking your games out, you can use the time to do the same thing. If for example a player's got a good win rate with a Sicilian defense, it might be a good idea to start with d4.