Thank you for your post!
We'll need some more information on this one too:
-What exact version of FileMaker are these clients running? (ex. FileMaker Pro 16.0.2)
-Does this issue occur in a New File?
I was unable to reproduce any slowness with FileMaker Pro 14.0.6, 15.0.3, or 16.0.2 when Specifying a Calculation for a Set Variable Script Step. Please provide any further steps you might have to reproduce.
You might also see the recommended template on Report a Product Issue for the information to provide on future reports.