Then they started getting XPS Print Error: memory allocation failure errors while printing. Which seemed unlikely on a printer with 1GB of RAM (ex-business printers are awesome!). A crappy old HP with something like 16MB RAM never got XPS memory allocation failure errors while a business-grade printer did.
After much poking around reading the usual reboot/reinstall/power cycle advice I found the thing that works: XPS stands for XML Paper Specification, a toxic combination of XML and Microsoft technology. To fix XPS errors, go to your vendor's web site and download a non-XPS print driver and the problems will go away. In this case no more print errors and printing back up to full speed.
The killer here is that Windows will try and install an XPS driver by default. So if you're offered a driver and you see the term "XPS" in it, you need to explicitly install a non-XPS driver that actually works properly.