Hi, Jürgen

I see no files inside dir ParallaxImage\jquery.scrollspeed in ParallaxImage_1_1b5.zip

But in file ParallaxImage\ParallaxImage.php we have

    $page->head_js[]  = $addonRelativeCode . '/jquery.scrollspeed/jQuery.scrollSpeed.js';

Is it an error in adding files or in linking script?

Upd.

I had a problem with scrolling on b3. After upgrading to b5, it is gone. But I see that the scrolling is smooth only when I use the scroll bar. When I use the mouse wheel or keys, the background image has a lag in scrolling and makes jumps.

Upd. 2

The height of the parallax container is almost the entire page. So I see this problem on your demo https://typesetter-addons.grafikrausz.at/Parallax-Image-Fullscale as well.

The background really yerks a bit too much.  But perhaps a change with the use of  https://github.com/pixelcog/parallax.js ( Demo ) - which also exists as a wordpress-plugin  - can help. Another solution is at https://materializecss.com/parallax.html ( DEMO )