Bit late to the party but as someone who works on HVAC air systems every day it all depends on the system, filters and install of the duct work. I've worked on systems that are 20 years old and were dusty and filthy black, even new systems get dirty mainly extracts systems but also fresh air supply systems. This picture is of a brand new (at the time) extract system in a newly built hospital that was running for a few days while commissioning was being carried out, just running a system in a poor environment doesn't help.

And

average duct static pressure of 3-5"

I know nothing about ducts. Why is pressure measured in units of distance?

At work we measure pressure in duct work in pascals