In Flash, If many elements are animating at once, if partial transparency is applied to all of them, if they are changing shape, if they cover a large area of the screen, and if MP3 audio is decompressing in the background while a download is simultaneously occurring, then slower processors can definitely show the effects of all these requests. Video can deliver a more predictable frame rate. Video codecs are time-based animation streams, and most movies will drop frames if necessary in order to maintain the desired frame rate. In event-based animation systems, such as Flash and Director, each frame is usually guaranteed to display, and time will be dropped in order to ensure that each frame is seen. Audio synch is usually simpler in digital video.
Video can usually offer greater detail at lower cost.
Some codecs will display a single bitmap more economically than JPEG, and many codecs can use interframe compression to make a bitmap stream more compressed than it would be as individual images. If you're dealing with much captured imagery, then a video codec will usually give smaller files. It's easier to scale bitmaps and apply effects in video. In a digital video file all resampling is done at design time, and so more expensive algorithms can be used than in realtime scaling. Although the Flash Player can resize bitmaps, to do so in realtime usually results in some quality degradation and a definite performance hit.