Why does File, Save As take so long on my iMac?

For the last couple of months, doing a File, Save As… from every application on my iMac has taken a very, very long time. An annoyingly long time — often as long as 15 seconds.

The delay is between the select of Save As… from the menu and the appearance of the dialog box that lets me choose the location for the save; once I actually enter a filename and click Save, everything proceeds speedily.

I originally thought this was because I had drives on my Linux machine mounted on the iMac, but I’ve eliminated this as a possible cause by experimenting without these mounts present.

I’ve made a movie that shows just how long it takes, using BBEdit as an example.

I have to assume this isn’t normal behaviour, but I can’t for the life of me figure out what’s wrong. Some notes:

• I’m running Mac OS X version 10.4.3, but the problem predates this update, and several previous.
• The iMac is on a local LAN behind a Linksys router.
• If I unplug the Ethernet cable from the back of the iMac the problem goes away; when I plug the Ethernet cable back in, the problem returns.
• Personal File Sharing, Windows Sharing and Personal Web Sharing are all turned off; Printer Sharing is turned on. The Mac OS X firewall is turned off.
• On occasion I mount Samba shares from Linux machines on the local LAN, but the problem occurs whether these are mounted or not.
• Nothing shows up in the Console Log when I try to save files.
• I’ve repaired disk permissions using Disk Utility several times; no effect.

I’d welcome any advice from Mac-literate readers about what I should try next.