We don't test or support other servers than Apache. On IIS you are on your own and dependent on users that might have any idea about this. Others do reportly run Zenphoto on IIS or lighttpd as well.
Also Zenphoto is not a slideshow. You might also read on the troubleshooting how Zenphoto works and that it generates the image size for display once on request. Depending on the size and power of your server that might take some time. But afterwards the images are cached. Some recommended read:
http://www.zenphoto.org/2009/03/troubleshooting-zenphoto/#11
http://www.zenphoto.org/2009/03/troubleshooting-zenphoto/#22
You can try it for yourself if you have FF / Chrome / IE8 installed (or perhaps other browsers as well).
Here: http://www.familybynet.net/gallery/
It's just a trial site at this point - but you can see if you get the same results.
The performance is a little slow, but it is the same for both IE8 and FF for me on the initial load. FF is faster on already loaded pictures. In fact I would guess that IE is re-requesting the image from the server as the re-load seems the same as the initial load.
This does not happen for me with my site. (Try http://testalbum.sbillard.org/Glapagos/20080425_122531-slb.jpg/view.) so maybe there is something in your server configuration that is fooling IE into re-requesting the image.
Thanks for posting the comparative site. Yours does appear faster with IE8
On another test, there was virtually no difference using a Macbook Pro with FireFox. Although, at times, I could detect that you have interlacing turned on.
I'll try testing again later with a Windows laptop pc.
I'm getting the same results on my windows laptop. Both test sites run pretty good on IE8 with yours nosing out mine most of the time. When viewing on FireFox, my test site is very fast, maybe nosing out yours.
Dunno'. . .maybe it has something to do with virus protection security too . . .
Anyway. . .I'm very pleased with ZenPhoto, even running it on IIS and look forward to trying its many features.