Aperture very slow loading the next image...

OK, I've trawled the forums but I can't find a specific answer.

Firstly, I'm using a low spec iMac (2007 2.4GHz C2D with 4GB RAM, Radeon HD 2600 - 256MB VRAM on 10.8.2) but I'm not convinced this is the issue.

When I navigate through photos (Sony Alpha and Canon EOS RAW) there's a major delay of up to 20 seconds when the image shows "Loading" with a spining gear wheel-thing (not a colour wheel). The delay is worse on the 17Mpixel Canon files.

I've check what the activity monitor is doing during this and the memory seems fine, I still have over 1GB free and no pages in/out or RAM activity of any kind. The CPU goes ballistic - 180% on Aperture - it's really working. Given the file size/delay difference, I'm assuming the image is being generated with adjustments composited etc.

I thought the previews were supposed to address this by providing a cached image so my question is; is there any way of checking the previews are being accessed? I've deleted the previews from a selection, re-generated using a couple of size setting in prefs, they've been generated but still no speed-up.

I've also done a permissions repair both in disk utility and cmd-option launch of Aperture. No difference.

Thanks for any help in advance, Dave