How fast the platter moves is determined by the motor speed and wheel size. You can slow the table by reducing the outer diameter of the rollers. The plate rotates faster than the spider because it's driven by the wheels, which roll on the table top as they're pushed from the center by the spider.
You could make up for the smaller wheel diameter without altering the motor and center bearing height by building up the raceways on the tabletop and underside of the platter.

Neil