1:
Set System Clipboard to an UUID：4NQ2L-OMVNY
2:
Do some operations on the clipboard.
3: (here's the point)
Pause Until
Which is technically faster?
Welcome. Don’t know.
Your avatar?
Nature yields secrets only to experiment, and even then reluctantly and uncertainly,
(so an experimental design would take the question further – perhaps large multiples of each action type in sequence, with time capture before and after the condition test and branch ?)
but in this case my first guess is that the bulk of the time is used elsewhere, and that little of substance will turn on this choice.
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Incomplete information. Do what operations on the clipboard? After all, if you are setting the clipboard to %SystemClipboard%-A, your 3rd case will be the slowest. But if you are doing \L%SystemClipboard% the 3rd case will always be the fastest 
DUCY?
But while technically interesting, I'm with @ComplexPoint -- it really doesn't matter which you do. Pick the one that's most appropriate for the macro's logic and your way of thinking -- some people are more comfortable with "wait until it is not", others prefer "wait until it no longer is".
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