This site does not scroll smoothly at all!

How come? This is the site www.jivanmulders.nl

It isn't the large background image. I removed it and the scrolling experience stayed the same. It's in all browsers ...

I suspect it's the HTML ... but what and why? Thanks for any help!

Omit the background image and all these "jivanmulders_top_A.gif" etc graphics as well. The jQuery localScroll/scrollto seems to work smoothly without them.

It is the fixed background image together with the transparency effects. Being fixed mean that the browser has to re-render the complete page every time you scroll a single pixel.