I've read in many places that you should change your furnace filter
regularly as a dirty filter will lower the efficiency of your furnace.

Excluding extreme cases where a filter is exceptionally dirty, how
much of a loss in efficiency are we talking about for the average
"needs to be changed in the next couple of months" filter?

PS: I'm not trying to find a reason to stop changing my furnace
filters or anything. I'm simply curious as to how much impact a dirty
filter can have. Is it 0.1 - 2% or more like 10%?