MJPEG File plays very slowly

[felix6969]

By transcoding a youtube VP80 Stream, I generated a MJPEG file.
Generated at "normal" speed, i.e. about the same speed as playing the source stream.
But when I open the generated File, it plays at a very slow frame rate of about 1 fps.
Playing faster -> also 1 fps, skipping frames.
I did not find any settings that could be wrong - ?

VLCPlayer 2.0.8 on Pentium M 1,73 winXP, intel 915 Graphics
the same with VLC 2.1.3 on Intel Atom intel US15W Poulsbo Graphics

On both computers no speed problems (as far as I see) with other file formats.
Speed Problem only with MJPEG file.
At the moment I don't have a live MJPEG stream source.

What's wrong?

[felix6969]

Addendum: The generated file is .ps, according to MJPEG container format.
If I use avi as container, all is well.