I don't like being kept waiting. And the 10 to 15 seconds it was taking for to preview each email message in Leopard was far, far too long. Bare Bones head and founder Rich Siegel is a pal of mine, so my quality assurance email report was full of detail and umbrage. Rich suggested that Spotlight under Leopard might be engaged in some activity - Mailsmith 2.2 uses Spotlight quite effectively - and that I might see an improvement after a couple of days.
Drat the man, he was right, and he's a gloater. via Twitter: "@glennf Stay after class and write on the board a hundred times: 'I will never doubt @siegel again.' Hm. Might make a good 'Simpsons' intro."
But after installing QuickTime 7.3 and restarting the PowerBook, the problem recurred. I sent Rich some more troubleshooting data - a Sample Application report available with a button click via Leopard's Activity Monitor that pulls in tons of low-level detail about what a program is doing - and he saw the problem: PGP Desktop.
Although I didn't have PGP Desktop 9 launched, the pgp-agent process was still running. I disabled the item in the Login Items portion of my account setup in the Accounts preference pane and restarted. No good. Terminal showed me that when Mailsmith launched, several pgp-agent daemons would also appear. Mailsmith works directly with PGP's encryption tools; other mail programs tend to rely on AppleScript for integration.
I was unable to find instructions for uninstalling all the PGP components, and the company confirmed for me via email that I should have taken the wise step of uninstalling the program and its pieces via the PGP application before upgrading to Leopard if I was worried about compatibility.
Rich suggested I upgrade to PGP's to see if that solved the problem. Of course I did, and the problem went away. (See the tweet above.) With this version installed, I can also now easily uninstall the software through the application. The folks at PGP also said that  can provide an uninstaller script for those who don't want to run or even install a beta.
Upgrading PGP seemed to solve a host of mysterious other slowdowns that might have been related to Mailsmith's interaction with PGP, and my PowerBook no longer feels nearly unusable under Leopard. In fact, Mailsmith 2.2 (build 227) is notably zippier under Leopard than in Tiger.
I'd like to believe there's a moral to this story. Check all your software for upgrades and compatibility issues before moving to a new operating system? But I didn't think I was "running" PGP as the obvious application portion wasn't running. Perhaps the moral is "have a revert position in case of failure." Or just, "I will never doubt @siegel again."