I couldn't put it any better than FiberOPtics and qwerty.

It's simple enough to test these things for yourself
just by putting the code in an alias that times the parse time
of the while loop.

Just remember that your OS is sharing the CPU with other things too. So you'd really need to take an mean time after
running the alias a few times.

Keep in mind, its very important to have an equal number of true and false tests in the if. I'm using $and to check for odd and even numbers in the loop, so it'll be 50/50 true false with an even number of loops.

Yes, as it was said, you'll not notice much difference most of the time and it would be better to design faster code in the first place.

You can play around with the following code to test it out
for yourself.
Just change the if in the while loop to the code you want to test.

Code:
alias benchif {
  var %t = $ticks
  var %n = 4000
  while (%n) {
    if ($and(%n,1) == 0) { .echo -q %n }
    dec %n
  }
  echo -a $calc($ticks - %t) ms.
}