The argumentless-For  is the worst in showing intent of the code.

A quit statement can be anywhere in the code. There is no clear definition as to where to put the condition for ending the loop.

What is wrong with a while that shows clear intent:

s struct = $ORDER(TABEL(""),1,data)
while (struct'="") {
  s struct = $ORDER(TABEL(struct),1,data)
}

I don't like the fact that u use for loops to traverse a global.
In most programming languages for loops are for when you know the number of iterations needed in advance.

see quote below straight from stackoverflow:   http://stackoverflow.com/a/2950945

The main difference between the for's and the while's is a matter of pragmatics: we usually use for when there is a known number of iterations, and use while constructs when the number of iterations in not known in advance. The while vs do ... while issue is also of pragmatics, the second executes the instructions once at start, and afterwards it behaves just like the simple while.

I don't think the above code proves anything.

The first time you run trough it it gets cached, so the second time is bound to be faster.
Try to run another 2 full runs and another 2 zn runs and you should see they are way closer.