Be careful when interpreting fan motor RPM with a centrifugal fan such as that used for the Bullet exhaust. The higher the flow rate (the higher the cubic feet per minute) for a given centrifugal fan, the slower the motor RPM. Makes sense- move more air and the motor has to work harder.
Now the messy part. If you restrict the outlet of the centrifugal fan used for exhaust in the Bullet, this reduces air flow through the impeller and the impeller blades tend to stall, i.e. they spin without doing as much work like moving air. When the exhaust impeller (a centrifugal fan) stalls it stops moving air. With a completely blocked outlet, rotating speed of a centrifugal fan will increase to the maximum the motor can spin and air flow is zero because the fan isn’t doing any work. Unblock the outlet and the impeller starts doing more work by moving air… the load increases and the motor slows.
Compare that to the typical blade fan, e.g. the muffin fan in the Bullet cooling tray. Blocking the outlet of a blade fan produces maximum load on the fan and that causes motor speed to drop- exactly the opposite of a centrifugal fan.
The only real way to be sure what is happening when motor speed changes in the exhaust fan is to use a separate flow rate measuring device (and yes, that can restrict air flow through the exhaust fan). In the bad old days we installed a “feather” in the path of cooling air inside electronic equipment enclosures. The feather was a lightweight piece of plastic the was pushed aside by the airflow. The displacement of the ‘feather’ tripped a switch which enabled the rest of the equipment to power on; no air flow, no power to the equipment.
re: drop in exhaust fan speed with trier or bean chute plug removed… a drop in exhaust fan speed with either of these is removed is exactly what you should expect since there is less restriction at the inlet to the exhaust fan, so the exhaust fan is moving a tiny bit more air which puts more load on the exhaust fan motor.