It's vitally important to him that he finish as near enough to Watson as possible, and he keeps himself on edge for so long that he's trembling with the effort. He can hear the signs though, can tell when Watson's close, and he starts to let himself go. When Watson's orgasm hits him, Holmes is close behind, his hips stuttering as his mouth falls open, whatever sound he might've made caught in his throat.

He kisses Watson, insistently tender, while the last shocks of his orgasm start to fade away, and then he drops his head to Watson's shoulder. He shifts until he can hear his heartbeat, can feel Watson living beneath him, and he closes his eyes.