It depends on the design of the impeller blades.
Most vacuum cleaners speed up when the airflow is blocked.
But if the blades are of a different design, it could be the
other way round.

If you are a real geek, get a couple of probe thermometers.
Stick one in the intake plenum and one in the output plenum
and even amhigh temp one in the flue.
Mark down what the normal temps are and if anything changes
you know something needs fixed. Works for AC too.
M