Diffusion is precisely why open-air filters (filters that draw in a portion of the air and filter it) work!

The Dyson filter is an open-air filter, that draws a small sample of the air through a filter, and passes the rest through unfiltered.

Each pass reduces the average (diffused) load of contaminants.

Two identically effective filters with different flow rates will eventually achieve a specified target diffused contaminant load in the air in the space; the only difference will be the time taken to achieve this (given a fixed initial contaminant load).

This should be clear...