Sounds like the clutches are not receiving the fluid as fast as they should. I would suspect a valve is not opening all the way. I would try some trans fix or similar from the local auto store. If that doesn't work you can pull the pan and poke around and try to fix it. Or just live with it, after all how much as a faster reverse worth to you?

Usually if the clutches are worn out you would see their remains in the fluid and bottom of the pan. Worn clutches do take longer to engage, but shouldn't be very noticable (varies with vehicle). Most importantly; worn out clutches usually show themselves by slipping. A good solid engagement is a good sign, do you have it?