When you compare the smoothness of the animation of the .GIF file and using the play function and using the play function Within Pencil2D.
The .GIF being played using say a browser is much smoother.
What happens with the play function is that just after the bounce, with the ball squashing, for a tiny time interval both this fame and the next frame with the ball travelling upwards are displayed at the same time.
When the Ball-2.pclx is output as the .GIF file, the file consists of a series of 10 frames stored in sequence. When played back there is no processing being carried out, the frames are being displayed in sequence.
When the play function within Pencil2D is running, there are many background processes, such as keyboard scanning, reading the mouse position both to detect commands. There are also other processes of handling external files and other packages running in the background.
The best way to check the flowcof an animation is to export it to a .GIF or .Mp4 file and use a browser or external video player to play these files.
Exporting animations with sounds as an .Mp4 files is the only way to check sound synchronisation with the drawings.