An update: the computer where I am having the issue is running Windows 7 and PsychoPy 2021.2.3
I found Arboc’s response in this thread: Confusion about how to implement non-slip timing for trials with known end points - #8 by Arboc
while troubleshooting this afternoon and the idea of “tricking” PsychoPy into using non-slip timing led me to realize that even though the blue Routine block isn’t presenting any stimuli, it is a gap in the continuous non-slip timing of my loop. I figured out that the code components will show up as non-slip eligible (green) if I added a text component, then reset its time, and then deleted it again. When I tested that version of the script I still got some increasing delay (which shouldn’t happen with non-slip timing) but it was much improved.
My next test will be to just move the code components into the already-green routine rather than having (blue) routine that is all code. It didn’t occur to me before that the code component would affect the timing and I can easily put it in the main part of the trial instead.