I just finished installing a Samsung ML-1210 laser printer. It's supposed to support Linux; I'm using Fedora Core 2. RedHat's 'printconf-gui' tool's Add Printer Wizard listed a driver for the exact model. I installed the printer, applied the changes, etc. So far so good.
Now, I know it's a Winprinter / GDI printer, so performance will not be great. However, on an Athlon 650 MHz machine with 256 Mb of RAM, everytime I print so much as a test page, the system basically becomes useless while the print job is processed, taking almost a minute per page. Once the printer starts printing, it's actual print speed is fine, and the system performance is back to normal. So it works... But with this performance issue, it is not usable! Is it just me experiencing this, or is this something I have to live with to use a GDI printer on Linux?

I don't know if this will help (probably it won't), but check if you are printing directly to the printer or spool.
Printing directly may block your programs, spooling will not.

How can you check / change that setting in CUPS? I don't think CUPS EVER prints directly.