A reasonable compromise is achieved by an air purifier that has the outlet on top because it’s blowing the filtered air well away from the inlet side and generally circulates the air in the room.

I used to work in the cleanest room in the state, that wasn’t a lab, back when I lived in Adelaide, and top-inlet button-outlet in a raised floor data centre is how the air handling units in the main data hall worked, so you can simulate that to a certain extent.

> The more effective the filter the more it restricts airflow.

This isn’t the whole story. Surface area and flow rate are factors.

A residential home is aiming to improve air quality, rather than grow flawless silicon ingots etc.