I am having the same issue at work and have done the patch recommended on Brian's page as well as unplugging from the network. I did a test on my 2.3 GHZ PC at home and my 3 GHZ at work and got very different results.

Opening a 115mb file
Home 30 secs
Work 1 minute 15-45 seconds

It starts to open the file but the status bar pauses then Photoshop shows as it is not responding... thus the application turns white for about a minute and finally opens the document. I have just don't understand why it would be so slower on what is a faster machine. I have tried disabling the Symantec virus scan, but I am wondering what else could cause photoshop to perform so poorly. I have tossed the pref files by doing the ctrl + alt + shift... updating drives and de-fragging... any other suggestions?