I wouldn't say it is. Everything that we see before the game begins is an absolute necessity to the story and to the set up of this revolution that's on the brink of happening. I think perhaps a reason people might find it slow is because everyone is just waiting for the games; that is the big event that everyone wants to see.
All the stuff before the games impressed me so much. There was not one scene that had no reason to be there. Literally everything was necessary and Francis Lawrence developed the world and the imminent uprising so perfectly in my opinion. I wouldn't say it's slow, it's just not as fast paced as the games and rightly so, it shouldn't be.