It's pretty easy to do lag compensation on RTSs. You have the server run a copy of the game locally and just sync with the clients. Typically, RTSs have a lot of actions, but timing isn't as crucial. So, if things get out of sync, you just buffer until you everything catches back up again. You can even do things like vary the speed of the game (FPS) without the players noticing much. With games that rely on precise timing as a core mechanics (e.g. a shooter, fighting, or rhythm), the player is going to notice that an action took 10ms and then the next time they did it, it took 20, because they'll actually miss their target.