I'm a brand new user still trying to install my first site, so what I say here could be wrong, read it with a large grain of mistrust.
I experienced a worse situation creating a demo model using XP pro. And oddly it was slow even when the only connection was one person logging in at THAT machine.
My first problem was that Apache is set up to use an assigned IP address. So check to see if your server has a static IP address, and if it doesn't, get your administrator to assign it one. (This could also be why IIS would be slow.)
Secondly, reading the how-to's points to using IIS as a problem. It seems to me that Moodle is written with Apache in mind, and you have to be an IIS guru and tweak it alot, to get to run decently. So, I suggest you read, read, read, the documentation on this site about IIS. The tweaks are numerous, and spread throughout the forums and docs. (Did your Zend Core install Apache, so you have both IIS and Apache running at the same time?)
Next, I screwed up while installing the OS (I started with a new hard drive). The machine was fast, up until windows update loaded service pack 3/IE 8. Also, because I wasn't sure what was needed I had let windows update itself automatically and it installed: .net support, Silverlight, DirectX, SQL Server 2005 Compct Edition, Visual C++, MS Sync Framework Runtime Native (and Services Native), Media Player, MSN Messenger, Windows Defender, Windows Live (Sync and Upload Tool), and Windows Search.
To further cripple the machine, I installed Java.
As you can see, Windows went hog wild and each of those bits of software use ram memory and processor time.
If you don't have the expertise to uninstall the above, or you need it for other services on your server, I would suggest installing the mazimum RAM that you can, as with all the programs listed above, my machine starts using the swap file if I have only 1 Gig of RAM, even before I installed anything but Java. (Just in case you don't know a swap file is, when the RAM isn't big enough, Windows starts using the hard drive as RAM memory (simplified explanation) so everything slows down to the mechanical speed of the hard drive instead of the electronic speed of memory).
Last, I see you used Zend Core that includes an optimizer. I tried to find the reference about a php accelerator (but couldn't) that said it actually makes the performance worse. So try looking up accelerators. (It could be about Apache or MySQl)
Hope this helps. Good Luck.