A shorter whip takes away all your speed at the end of the whip!! Nooo, go longer, go thinner, go more bite so it hurts more when you DO use it. IF your horse is not lightening up like it should then your maximum disciplinary action is NOT strong enough!
I have had several lazy horses including one pony that was absolutely DEAD to leg aids, no matter what you did he would not go. The kids who rode him before I got him weren't allowed to ride with a whip so the first thing I did was get a crop. Ask lightly with the leg, then firmer, then a tap with the crop, and then I would bring that crop down on that pony's ass so hard I nearly fell off from the force of hitting him. It got better but with the crop I never got past the point of having to use it for a light tap. So I got a dressage whip, the longest and thinnest one I could find, and all of a sudden I didn't even have to put much force into hitting him when he was at his laziest/naughtiest. Didn't take long to soften him up. By the time I sold him, when I had that dressage whip on me all I had to do was touch him with my legs and he would go. But he knew when I didn't have it and wouldn't move unless it was there!
I have used spurs to reinforce my forward leg aids and found it does work, but that was on a horse that I couldn't use a whip on because he would bolt. I would much rather use a whip if possible, it's much more effective.