Why play so quickly?

In my last few games, which I set at 15/10, I noticed that all of my opponents played far more quickly than I did, and hardly used up half their time, even though they were full-length games (20+ moves each).

Why did they accept that invitation, if they wanted a faster game?

I won 3 and the other one was very close, so it is not like playing against me is trivially easy.

I'm not complaining, I'm just interested in people's motivations.

They did not " want a faster game ", they play at their best tempo/speed/pace.

Two minutes is too short for me ( rating around 1200 ), three minutes is better, but playing 5/2 gives the best result, rating now around 1400, so apparently I need four minutes play.

The answer to your question lies in the possibilities to choose between 2, 3, 5, 10, 15, 30 and so on minutes.   Your 15/10 opponents apparently need 8 to 10 minutes, but when they choose a lower tempo, they play less pleasant...

Moreover, you just noticed it and are not complaining, but both parties can have remarks : when I use 3 minutes to play a game, and the opponent 5, I often have to sit and wait for his/her move, I might think " two minutes nothing to do, " just waiting...; and my opponent can be irritated or annoyed...

Both reactions could influence the game, so : back to basics, play your own game in your own pace...