On cars with a cabin filter that only filters outside air, the blower pitch will change some when switched, not sure about speed but possibly I guess. I wouldn't think this to be much though as the HVAC case on that car has virtually no difference in the length of the path to the blower motor regardless.
And it seems to me that, from my many experiences with neglected cabin filters on many different vehicles, often times the blower speed seems to be faster when they are stopped up, but the volume is diminished, likely due to a low pressure situation in the air ducting. But that may just be how it sounds (you can do the same thing with a shop vac by blocking the inlet... that is pretty obvious it speeds up the motor dramatically).
In any event, the OP may have various issues that need to be sorted out, and the cabin filter may also be living in a cowl that is plugged full of tree debris, mouse leavings, or who knows what. The junk cars (not just B4s) collect in their plenum chambers is amazing. It seems like older cars that simply didn't have this cavity in the body layout had an advantage in that regard.
