Maybe it's not slow, a right has to travel further than a jab, use the jab in front of it, use the jab to blind your opponent, get them blinking, and even sneakily clear his left away for your right. Anyway, the delivery of a cross: straight from your shoulder, the straighter the better, do not draw it back, do not try to put extra power into it, twist your hips and your shoulders into it, snap your fist over at the end, and, most kickboxers have very poor delivery,(possibly because they are preocuppied with getting leg kicked). By boxing standards, most of your bodyweight should be on your front leg at the end of the punch, never over it, if you miss you will fall over, your right knee should be pointing straight down at the floor and should have very little weight on it by the end of the punch. Also, study the best, Tyson had perfect form at his best, watch how he threw straight rights. Ali had a right that won many fights for him, if you watch him throw his, he sort of brings it up from his hip and adds a beautiful shoulder snap at the end of it. If you are indeed slow handed, watch the very slow, Alexis Arguello who would throw the tightest right you could ask for and i've seen him beat faster guys to the punch just using good distance and timing.
