It is less than 1-second of delay, but instant response would be nice. Is this normal for a non-supercharged XJ? Any way to tune/adjust it?

I have driven cars with a much longer delay. Both late-90's Dodge Ram 5.4 liter and Mitsubishi Galant 2.4 liter could count one-one-thousand-two before the power would kick in.

A new '10 Saab Aero is like a Jag in restricted-performance limp-home mode every time you hit the gas until the turbo kicks in some 3-seconds later. That Saab is an excellent $15,000 car - too bad it sells for $40k

The only related throttle issue that I am aware of is to adjust the throttle cable where it operates the throttle position sensor. Many have tightened up the slack in the cable for a little more throttle response. Push down on the bare cable with your finger and see how much slack there is before the TPS moves. Needs a little but not much.

That probably doesn't specifically and completely address your issue/question but may have a positive affect on throttle response.