Porthos might not look it, but he is actually quite fast. He's not the fastest of them by far (he might actually be the slowest, though he'll never admit it), but he's more than fast enough to block that attack, and use only some of the force he would normally need to push Aramis back. "At least you can use your sword. You've a leg up on me, back then," he remarks, poking fun at himself now, and he steps forward into a simple attack, wanting to see how well Aramis can block in this body, which presumably has less strength than his normal one.