brief runs vs lengthy runs

[khumphries1]

I have a flow that syncs two SharePoint lists. The process is a bit inefficient, as it involves

• building an array of all items from a source list (approx. 4800 items)
• then building an array of all items from the target list (approx. 4800 items)
• then creating a intersection of the two datasets to determine what's already in sync
• then looping through the target array output to delete items not in the intersection
• then looping through the target array output to create new or modified items not in the intersection

but, it works!

I have two versions of this flow:

1) a Button flow

2) a Scheduled flow

The Button flow completes in 2.5 hours, give or take a few minutes.

The very same flow started on a Schedule requires anywhere from 12 to 36 hours.

Who can explain this?