Robert Green wrote:

Anyone still reading: Why DOES the motor RPM increase quite
noticeably when the hose is clogged. Does an internal vacuum relief
valve sense the blockage and open a relief valve? Since it would be
close to the vacuum motor, probably in the head, it would not have to
draw air from the entire piping system and thus the motor RPM would
soar because of the lightened load. Sound right?

Block off the air flow, and air just goes around and around with the fan blades.
Without the block, it is constantly accelerating air from the input to the
output, which uses more power.