Hi. Welcome to the forum. Normally,depending on the lay-out, a "set-back " of 5 degrees would take approx from 20-30 mins, or less. (This could be a burner stage problem). But, there are other various reasons why some furnaces take longer than another to recover a set-back. First, when your furnace starts this recovery, does the burner stay on the whole time.(75 min), or does the burner shut off periodically and come on a few minutes or so later? Post back with info and we will advise further

Hube