I believe - but cannot confirm - that the filters in the airconditioning ducts were not often cleaned or changed. This led to cockpits being very noisy as the air was forced through the partly blocked filters. And no doubt, a lower airflow to the cabin as well.
You are certainly more than right. At my airline, a group of pilots, cabin crew, joined together and created a whatsapp group to share all technical information regarding cabin air quality.
They noticed that the filters were not changed regularly enough at all, and removed some panels to see them on many planes, they generally saw them completely obstructed by dust.