why do :15 second games take so long to watch?

I started watching some 15 second games between two players, no per-move extra time (no fischer clock). But when I opened the clock on Windows I saw these games take like 48-50 seconds to complete.

Instead of 15 as would be expected. Why? They are almost 25-second games then, which is a huge difference...

Lag compensation time is added maybe?

expected for 40 moves with 200ms ping

If you are really long on time, lichess gives you some increment. I think it's 0.25 seconds or something, and some players are really good at premoving.

Premoves don't take any time off the clock, right? Surely that's the answer.

#2 is the correct answer. After a move, lichess will compensate for the lag when it reaches the server.

This basically means both clocks are stopped between a player has moved, and the other is waiting to see it on their screen.