I have a Hewlett-Packard Deskjet 3920 USB inkjet printer, using it with the hplip driver. I dual-boot this machine with Windows Vista, and noticed that, for whatever printing configuration I choose, in Linux the printer prints at about half the speed it does in Windows. Any idea...?

[Edit] My distro is Kubuntu 7.04 (Feisty), with kernel 2.6.21.5 (64-bit) compiled for Athlon 64 from kernel.org archive sources.

try to disable bi-directional printing.

How do I do (and undo, "just in case") that?
I believe you're not talking about printer communication with the PC, over USB it's always bidirectional, and in old parallel port usually printers did better with bidirectional communication enabled...

I had a similar problem with an Epson 740 using OpenOffice on XP and Ubuntu; the later being half the speed. You have two places to go:-

For Future documents you can change the printer settings by going to System/Administration/Printing; Right click on the printer, choose properties and the advanced tab. You have a selection of things to twiddle, including bidirectional, but remember the defaults. I set Print Quality to Draft. The print speed then equals windows and the quality is similar to but not quite as good as the windows settings. ('Fast economy' equals XP's 'Draft' quality).

I don't think this will change any documents already made. For this go to the document and find the Printer Settings hunt; for advanced. You should get a similar set of items to twiddle.

Looking at the printer settings in windows might help but the options may be quite different.