Pretty much anyone who thinks strength training will slow you down is an idiot. Strength is about neuromuscular coordination and not about how big your arms are. Look at it this way, you use a certain amount of muscles when you punch. With strength training, you teach your nervous system to recruit more muscle fibres in the same movement, or to use the muscles more efficiently - thereby improving strength and power. Olympic lifts are good for this, although not the only option. You can use resistance bands, medicine balls and bagwork to improve power. You can even just do normal strength training, while trying to lift the bar as fast as possible (known as dynamic lifting). There is no reason for a boxer not to train for strength, as long as it doesn't take the place of skill training or conditioning, it can only make you a better competitor.
