I just sorted out an odd problem with one of my printers (HP OfficeJet 4215xi) and thought I should post the results here in hopes of helping someone else.
THE PROBLEM: After months of working just fine, this printer suddenly started printing VEEERRY SLOW - like 3-5 minutes to print a simple B&W page that used to spit out in 15-20 secs. Worse, the problem was intermittant. Sometimes after running slow, it would jump back to its old zippy self for a while. Of course, as with most intermittants, the circumstances of these speed changes suggested a bunch of possible causes that were totally bogus.
THE SOLUTION: Long story short, the problem turned out to be caused by a defective black ink cartridge (HP factory) I'd recently installed. I've probably owned or used at least 10 different HP Inkjets since the original DeskWriter, and I've never run into this before. And you wouldn't normally suspect the cartridge, since once a slow-printing page came out, the quality was perfect. Nothing on the page to indicate a cartridge problem.
A KEY DIAGNOSTIC HELPER: After hours of chasing bogus clues through the driver software, the USB connection, etc, I realized that there's a way to tell for SURE whether the problem is in the computer or inside the printer itself: The test page to align the ink cartridges is generated entirely inside the printer. You can print it out with no computer connection at all. Sure enough, when I tried it, the alignment page printed out just as slow as everything else. Aha! From that point a little quick experimenting with swapping cartridges established that the slow print problem was being caused by that one bad black cartridge.
Worth bearing in mind if your printer starts making you crazy.