Post update / file upload performance

We are experiencing sporadic performance issues when uploading files or updating posts.

The poor performance does not occur for everyone. For all tests, we used the same user id and site. The only difference was the computer where the test occurred and slight differences in browsers.

From 2 computers we can access a site, upload files and update a blog posting with reasonable performance.

From 2 different computers, we access the same site and same blog posting and the update takes 3+ minutes.

These results are reproducible on the various computers.

All of the computers are on the same network.

For the computers that had problems, we tested with many different browsers IE9, Firefox 8 & 9, Chrome. In all cases, we had consistently bad performance. I also cleared cookies before testing.

The slow performance is only during the file upload process or when updating a blog posting. Performance is fine when navigating in the dashboard or viewing site pages/posts.

Also, we monitored the Apache access and error logs and do not see anything out of the ordinary. The sporadic performance issues do not correlate with heavy load on the server.

Does anyone have suggestions to find the root cause of this sporadic performance issue?

WordPress version 3.2.1
Network enabled
Plugin network enabled: WordPress MU Sitewide Tags Pages