Sometimes I'm racing and my car loses a little velocity. Why is that happening, could it be a internet lag issue? The other thing that happens a lot is that for example I'm in the 1st position during a long straight route, and the car in the 2nd position reaches me. We both have the exact same vehicle (no custom allowed), and I'm going max speed, how come he can reach me?
It's called Catch-up. It's a race setting that speeds up the players in back and slows down players in front. It can be enabled/disabled by the host in the lobby.
Also players close behind you can slipstream for a temporary speed boost.