HEPA filters remove  a relatively large fraction of particulates, depending on the grade they usually remove, in reality, 90 to 99 percent.  However what matters most for the ability of an air purifier to clean the air in a room is the CADR, the Clean Air Delivery Rate.  

  Clearly, if you have say 100 Cubic feet per minute with 80 percent particulates removed after it exits the machine, that's equivalent to 80 cubic feet per minute of completely clean air, plus 20 that is unaffected.   The unaffected air does not affect the concentration of particles in the room.  The clean air dilutes them, of course.  This is what CADR measures, the clean air fraction, per unit time. Another way to think of this is how much particulate matter the machine removes from the room per minute.  Obviously, more is better.

You can still get very large, and just as large, reductions in particle concentrations because the air in the room passes repeatedly through the filter.  So if say 70% is removed every time, and it passes through 3 times, that's 0.3*0.3*0.3 = 0.027 or only 2.7% of the original particles remain in the room.  97.3% removal.  Not bad. In reality the air mixes in the room so it's not quite like this, but that's the general idea.

There are several different CADR measures, because filter efficiency is different for different particle sizes, and to an even lesser degree, the type of particle.

Some people think that you need HEPA to trap smaller particles. This is actually not true.   There are many, many filter efficiency grades, and some of the lower efficiency ones work even better for total CADR compared with actual HEPA, given the realities of how fans work, even for the smaller (and smallest) particles. Filter efficiency actually rises, for the most part, for smaller particles, after a certain point, because of the wacky physics.

Air filtration is not like mechanical sieving, in which particles smaller than the holes in the sieve can get through.  Water filtration is mostly like that.  Air filtration is not.  In air filtration electrostatic forces are very important, and smaller particles are captured by the filter through these kinds of forces attracting them to the fibers of the filter media, and how they stick to them, etc.  

MERV-13 filters, as used in the BQAP, remove about 80-85% of particles in standard tests, in the size range where viruses and bacteria arise (0.5-4 microns, approximately 90% of sars-cov-2 was in there, the rest in larger particles, and almost none in the smaller ranges).

You can also get MERV-16 or even HEPA filters in the same size and shape as the 20 by 25 by 1 inch filters typically used in a BQAP.  Some combinations (possibly lower MERV rating, actually), may produce higher CADR in a given machine that is otherwise the same, this remains to be elucidated.  Different brands may also give higher CADR due to less resistance to airflow (sometimes due to higher pleating density, for instance),