As others have said, it really depends on the restaurant and the time. We have sometimes had to wait a really long time (30+ minutes) or were seated immediately. It's really hard to tell. Overall, we try to get to the restaurant about 15 minutes early, as recommended.
At CRT, I would get there early because you have to check-in and get your picture taken with Cinderella and then still wait some more to be called up to the dining room.
As for the lag between ordering and getting served, it isn't anything abnormally long or slow. I would guess between 10 and 15 minutes, depending on how busy the place is and what you ordered.