You got it! The speed impact turns out to be hardly measurable and I love the tractor-crawler as much as you do. It well may be a unique feature of newLISP.
For the uninitiated:
The tractor-crawler code pattern runs infinite without looping or recursion by means of self-modifying code. Statements get continuously appended to the function and popped of at the beginning. The change necessary was additional stack cleanup in the macro block.