Short answer
As the filter holds back dust, its surface gets dirty and airflow drops. Regular cleaning keeps the filter working close to how it did on the first day.

Vacuum
With it fitted, vacuum the outer surface on low power with a brush attachment.

Take it off if needed
Peel the hook-and-loop filter off the tape; for a framed screen, take the screen off.

Rinse
Rinse gently with lukewarm water. Do not use detergent.

Dry and refit
Dry it in the shade, then put it back.
Made to measure: hook-and-loop or roll
