Normally a slow eccentric is a sign of not being confident of your bar path. Try chalking the middle of the bar and wearing a dark shirt. Check your shirt after each set: you should have one, thin line. If you have a thick line or worse multiple lines, there's your problem. Once you have grooved that bar path, speeding up the eccentric shouldn't be an issue. I will add that I'm not a fan of an overly speed descent on the bar, you still need to control the weights.