There's a lot more to being fast than having super strength. In fact, a lot of guys I've seen in martial arts and other various fighting arts who are the fastest, aren't the strongest by a large margin. The sort of strength it takes for Cloud to wield that Buster Sword oh his wouldn't translate into him being faster. If anything, he'd be significantly slower than if he was used to training with a lighter weapon, even if he had super strength. Cap is the peak of human capability, which means not only is he strong, but fast as well. And even if Cloud batted him with the Buster Sword, as long as it hits Cap's shield (and odds are it will), Cap can stand there and take it with ease.