Tim Wescott fired this volley in
:

I'd be interested in hearing truth from one who knows.

The speed is dependent upon the mass of air being moved. It's pretty
large when sucking and blowing freely. The motor does the most work
moving the largest volume. When the inlet is blocked, the fan just
recirculates the air in the volute, and it's (essentially) unloaded.

The air isn't 'rarified' enough to make a huge difference... most only
suck in a few tens of inches of water column... and it takes almost 34'
of water to make one atmosphere's worth of pressure.

Lloyd