Cast members that move on stage require more CPU calculations than do stationary cast members. Each time a cast member changes position, the Movie Player calculates the new location of the cast member, restores the stage appearance from the prior position of the cast member, and creates the correct depth perception when a cast member passes behind or in front of another cast member or background visual.
To minimize processing requirements, move as small an object as possible. For example, if a door on a house opens and closes, don't define a series of house cast members with different door positions; instead, define the door as a cast member. The house can be a QuickDraw image or another cast member.