Restrictions on 90 Degree Elbow for a Bathroom Exhaust Fan?
The squirrel cage fan relies on velocity, not pressure, to move air. A near elbow stops the momentum and reduces airflow rate. If you need 90 degrees close to the fan, put 2 90s together and swivel the joints to make it into 2 45s.
Down the path a 90 elbow is fine. If the run is long or has many bends, use a double 45 or try to keep the path as straight as you can. At the outside, find a vent cap that seems less restrictive. The flapper valve in the fan should open and close freely. You can often hear it drop closed when you turn off the fan. Wait a couple of seconds til you hear it.