I thought you were thinking about replacing them with fast!
I would say slow over fast because you dont want to spike your insulin levels untill after your workout with your post workout shake. This basically opens up your muscles to flood them with carbs and amino acids anc creatine. Slow carbs give you a steady flow of energy for your body to work with and you will be using muscle glycogen for fuel. One thing to try is a caffine suplement! before a workout. I dont know how long your in the gym for but im usuallyin for 1.5 hours. i have a high complex carb meal at 4pm and then a flapjack at 5pm with about 1 litre of water. i hit the gym at 6:30 and im always pumped with energy!