Formula execution on background SFMs is not on roadmap immediately for web delivery. However on FSA tablet this will be supported in upcoming 19.2 release.
If we have customers looking for this requirement then we would revisit this early for web.
Just to understand more on the requirement, are you looking at separating formula execution and interaction on SFM separately i.e one SFM used only for debriefing purpose and another SFM configured with formula is executed in background.
Is this requirement proposed only because of long loading times.
Would like to understand how this will be used in your work flow and benefit you get.