I am a newbie at kiting, although spent years flying hang gliders, and, as a professional big boat sailor. My question is, what is the major difference in performance if all the lines are radically shortened?

Hey, welcome to the sport!

Short lines react faster to input, because of less lag picking up slack in the lines. The shorter lines the less drag they produce, so again this speeds up kite movement, and reaction times.

As im sure you'll already know from your wind sport experience the wind closer to the ground is slower and more turbulent because of friction, so long lines help the kite catch more wind, which is faster and smoother. However because of the reasons above it means the kite turns and reacts comparatively slowly.

An easy way of extending the wind range of a kite is getting different line lengths. Short lines for high wind, longer lines for low wind.
Average line length for kites is 20-25m. Racers etc use lines ~15m, and if you want a really low wind kite, line lengths can go up to ~80m.

Hope this helps, I'm sure it wont be long untill some of the more knowledgeable guys let you know some more specific stuff.
Tim

Elf' explains it well.

With short lines your window becomes smaller. Reaction becomes quicker. This can be handy in high winds. Less of a power zone to get through.

With longer lines you get a bigger window so bigger power zone. + the wind is often better up high so long lines are good for low winds. Control of the kite is less direct.

Changing up line lengths helps you get more out of a single kite but it doesn't compare to 2 kites.

Thank you both very much. Another mystery solved!