Among common household appliances, things one can make from stuff in the garage or hardware store, and reasonably safe, e.g. within reach of hobbyists and high school kids entering a science fair, what is the highest speed of motion of any bit of matter one can obtain?
The bit of matter, or surface points of some object, should be macroscopic - big enough to see, time the motion of somehow, make collisions with other bits of matter. Electrons aren't big enough! Fluids are fine too.
Guns, dynamite would be too dangerous. The tips of a fan blade or shutter edges in a camera are fine. Using a drill to make something spin real fast might be okay if all mad scientists involved are careful.