The trick is to use high-airflow MERV 13 filters (ideally 3M 1900, which has high pleat count and low resistance) instead of high-resistance HEPA filters, so axial PC fans can generate enough pressure.
For a standalone air filter which is pushing the filtered air into the same room its drawing from, we can assume reasonably good mixing of the intake and exhaust air. Thus to be equivalent to a HEPA-filter it only needs 1/3rd more airflow... Which really isn't much.
This doesn't follow for a a fancier setup where you're taking dirty air and blowing it into a cleanroom or something... but that's not how we're typically using household air filters.
And that really does make a huge difference to someone who has allergies.
IQAir does HyperHEPA, and they go down to 0.003 microns (so, a hundred times smaller, down to the size of viruses), and they capture 99.5% at that size. That’s why they’re used in hospitals.