Vector based graphics are represented by a plethora of data that must be computed by the CPU (vector points, arcs, colors, etc). Anything other than simple shapes with minimal points and straight lines will consume vast amounts of CPU resource.
There is a "Cache as Bitmap" flag that can be turned on. This flag stores the result of drawing the vector based DisplayObject for much faster redraws. The pitfall of this is that if there are any transformations applied to the object, the entire thing needs to be redrawn and re-cached. This can be slower than not turning it on at all if there are frame-by-frame transformations applied (rotation, scaling, etc).
Generally, rendering graphics using bitmaps is far more performant than using vector graphics. Libraries such as flixel take advantage of this for rendering sprites on a "canvas" without reducing framerate.