The result is sputtering pictures and sound that breaks up. Good digital movies have an even data rate. This rate is called the frame rate of the movie. As the frame rate increases, the data rate increases. To compensate for larger frame rates, either smaller windows or more compression is required.
More compression distorts the images.
Websites are especially demanding for low data rates. Movies must be small and compact to download in a reasonable time. If you wish to display fast motion, you may be forced to poor quality pictures in small windows. If you are showing slow processes, you can use one second slides with excellent quality pictures that cover the entire screen. In between you make compromises to best display the effect.