This question is answered in two ways:
1. Over time, HEPA filters actually get more effective at capturing tiny particles. As a HEPA filter gets used, it gets filled up with dirt and particles. This improves the HEPA filter’s ability to filter out the air.
2. As a HEPA filter gets more and more clogged up with particles, the resistance to air passing through the filter also increases. This means that although the filter is better at capturing particles, it’s more difficult for air to pass through it.
Overall, these two factors mean that a HEPA filter’s overall ability to clean a room (or the air purifier’s airflow and CADR) does decrease over time and with use.