How to speed up reaction time in Settlers of Catan?
One way to speed up the game is to increase resource production. There are two common approaches to this:    


During setup, each player's second placement is a city. This gives a boost to everyone's economy/engine from the start.
Play a 3/4-player game with the map layout for 5/6 players. There will be many more spots, more valuable spots, and less blocking throughout the game.


Obviously, neither of these addresses the issues with turn length and trading. It sounds like your friends need to hurry up and make a decision!