A Broom of Flying has a flying speed of 50 ft. (30 ft. when overloaded).
Can a creature combine their own speed with the broom's, for example by running 30 ft. on the ground before using the broom's 50 ft. for a total of 80 ft. of movement?
What if the creature has a flying speed? Can they use the broom's 50 ft. flying speed, then supplement the broom's speed with their own flying speed?