I found the same thing happening on my wife's MBP recently. I was not really too surprised as it gets next to no routine 'cleaning.'
I suspect it comes from bloated caches, particularly those that maintain the folder data. That could be the infamous ".DS_store" files inside every folder. They are invisible (because of the leading ".") but there are many apps/utilities that can delete them...until the folder gets opened again. But that very task may, indeed, help. But it could be extremely time-consuming with the normal system with hundreds of thousands of folders!
What I ended up doing, during various tasks in preparation for upgrading her to Snow Leopard (from Tiger, BTW) was to run Disk Warrior's directory rebuilding function. I intend to also use TechTool Pro's volume de-frag function, also. While those too tasks are probably not directly related to this slowness, it may be increasing the access to the actual images when they are requested to be displayed. those files must be read into display memory from where ever the data actually resides.
Her folder displays normally now but I cannot positively define any of my work as the exact cause. OTROH, none of the above should prove damaging,
as long as you have proven, bootable back ups!
No doubt some one will come along in a few minutes and provide a link to some completely simple, effective and proven method.
The above is simply my personal observations. YMMV.
Another possibility is to simply remove 99.999% of your images (and other files) from your drive.
