1: Club length
3: Size and physical ability
Clubhead speed is a combination of mechanics and relative core strength IMO. Mechanics probably have the most to do with it though. Coordination is more important than strength by a lot, and you need more stability than flexibility, IMO. A compact but really on plane swing can hit the ball much better than a long but loopy swing, and it takes fewer compensations and less reliance on timing.
We take the club back and start it on an arc on the downswing. That arc needs enough length to allow us to accelerate it fully, but it needs to end up on the path we want so we can hit the ball consistently solid and straight. The farther off plane we get will require the clubhead to either change its path through the ball or for us to compensate by swaying back, missing the ball, casting, or rerouting the club to name a few problems.
265 isn't pounding the ball either, it's pretty good for a weekend player but a guy who considers himself a good athlete should be able to hit it over 280 if they wish if their mechanics are good.
Hip rotation is important but learning to slam your hips open is the wrong way to go about it; the best guys create that rotation as a by product of swinging hard, not creating the speed as a by product of the hip movement. You do need to shift your weight and you need to use your legs to help drive the rotation, so the hips need to move a lot and act as an indicator of weight distribution when seen on video. Same thing with "lag", that's not something you simply add to a bad swing.
Taking a long backswing is only going to hurt most players. I'm a very flexible player but I only go to parallel when I'm about to kill the ball or I want to hit it extra high. Making a longer backswing with a near perfect swing plane is too inconsistent. I can stay on plane on an insanely long swing if I lift my left heel and take my eyes off the ball, I could probably hit the longest drives of my life if I started to do that regularly, but it is less controlled and it can be hard to aim. At a certain point, you have to cheat a little because you're off balance and can't make the swing in a single athletic motion. Stop turning before you reach that point. For me, it's my vision; before my eyes start to pull off the ball I change direction.
Don't flail your wrists, btw. The wrist action in the swing should be a subtle one unless you want to hit down too much or hook it. I find wrist and arm strength to be more about control than clubhead speed for me.