As it turns out, two informed sources sent me related explanations describing what they believe is going on. One even provided a possible work-around (although you would have to be willing to take a soldering gun to your Mac's logic board to do it); in essence, the problem is associated with a weakening internal battery. For the full details, check the just posted MacFixIt Extras web page on the startup delay.
Last time, we posted an update on the issue of the startup delay that affects certain Mac models running Mac OS 7.6.1 or later (especially those with Tanzania and Alchemy motherboards, such as the Power Mac 6400). Apple claimed to have a fix in the works, but did not otherwise describe the the cause of the problem.