This week, we had an issue with Excel files opening slowly on the network. Some, despite being only 21K over 1000BT, took as much as 5 minutes to open. They opened fine locally.
Someone beat me to posting this on one of the sister sites, but since there are so many fellow system admins here, and I don't see it here also, I thought I would post the question and answer here too. I am sure I will save someone some time.
The problem was the Office File Validation, which was pushed via Windows Update this week. I was able to introduce this problem on a separate network. Removing Office File Validation solves the problem.
This patch was only for Office 2003 and 2007, so 2010 is not affected.
http://support.microsoft.com/kb/2501584
MS KB update: http://support.microsoft.com/kb/2570623