Some of that is just a misperception. If I remember the numbers correctly, in management science, they calculate with 15-25% of non-productive but still paid employee time for on-site jobs, depending on the type of position and work. It’s for stuff like toilet breaks, coffee machine chats, waiting for a meeting to start etc… People are often not aware of this when switching to home office work and try to clock full eight hours of screen work per day. That’s especially true when they do detailed time tracking with software, because then even the smallest breaks are excluded. Instead, 8 h × 80% = 6:24 h of screen worktime per day would be a rough equivalent of 8 hours at the office.
(I guess this should also go into the Distributed Collaboration Manual …)