In the concentric, my general attitude is to lift as quickly as is practical -- moving the weight forcefully, while maintaining technique. In reality, as it's already been pointed out, this might be fast, it might not. Often people need to start out slow while developing control of the weight and control over muscular involvement throughout the lift, so it may have been quite valid for you to have started out slowly, however more speed (if your technique is good with that speed) may also be good.
On the concentric, higher speeds help to build up the stretch strength reflex, while lower speeds mean more time under tension. Again, control is important here, whichever way you go. If you're just dropping the weight, then you're only doing half the lifting that you might do otherwise, however slowly lowering the weights can result in less reps. On pulling exercises and overhead presses, I like to do a controlled but fast/somewhat relaxed eccentric on most reps so that I can get more reps, then give a slow eccentric on the final rep so that I'm getting more work done than if I just dumped the weight. Doesn't work so well on squats and bench presses, where the concentric is the final part of the movement.