I ran into this recently on my MacBook Pro as well (including the notifyd spikes). Neither of the suggested options above are turned on (show full path, or rescan folder for changes). I thought "Aha, Capture Time", so I switched back to filename to test, but it's still spinning. Happens all sorts of times/places. Happens when closing the Preferences window. Happens when starting PM with no folder selected.
To generalize, it definitel seems to have something to do with switching away from the application. I was able to open/close the Preferences window several times in a row. Then I switched to Firefox to write that down here, and went back to find a beachball party. If I'm careful and don't do anything funny, I can double-click an image and scroll through handfuls of photos without issue, even hitting "esc" to return to the contact sheet. If I delete an image though, I get a beachball. Even switching back from Firefox to hit cmd-Q gives a beachball before it quits (about 30 seconds).
I'd be happy to provide a sample from System Activity if that would be helpful.
OS: 10.5.7, 2.2 GHz Intel Core 2 Duo, 4GB RAM. PM Version: 4.6.1
Folder contains about 1400+ CR2+JPG images (almost 3000 actual files) from Canon 5D Mk II and 30D. I can open the same folder on my mac mini without any problems.
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