I've had this problem as well and fixed it by converting the iweb images to one image. For example, you have alot of the rounded square shapes on your page used as backdrops and lined shapes as buttons. If these shapes have effects like shadow, lines, reflection, etc it can slow down the IE drawing when scrolling up/down. Each shape that has a shadow adds at least 8 additional files the browser needs to draw from including the shape itself making it 9. To reduce this.
- Format the shape how you want it with lines, shadow, etc
- Select it, Copy it using command-C
- Open Preview or any other image editing program
- Past it (Command-P). The shape should be exactly as you formatted it in iweb.
IF you want to give the image a name save it.
- Save as a png file if rounded or you're using shadow. If it's a square box with full opacity and no shadow then you can save it as a jpeg. The reason for the difference is you don't want a rounded box saved as a jpeg because a white background will be added (unless of course your site background is white then it wouldn't matter).
- Drag that file to iweb. You may have to resize it
IF you don't care about the name then just select all of the image and copy it from the image editor and paste it into iweb instead of the above saving information.
- Publish. Now that shape only requires the browser to grab 1 file instead of 8. You can test this by browsing your site directory on mobile me or publishing it locally and running get info on the page folder. The number of items should be reduced significantly. Not only does this make your site scroll smooth but will help it load faster and reduce the site size in Kb.
Also note,
If you are not using shadow and lines from above, it may be because you have ALOT of shapes in general going on. The backgrounds, the button boxes, etc. You can combine these into one image or 2 separate images by selecting them all then doing the process above. Therefore instead of the browser calling for 8 rounded separate shapes and 5 square boxes with 8 line files (the side and corners) each. It can call 1 background image (3 brown rounded boxes) and 5 square boxes.