You've probably got Quiet Mode turned on. It's a feature, not a bug.
Quiet Mode reduces printing noise by cutting the print speed from up to 30 ppm to 13 ppm. Check the control panel or your print driver. To turn it off via the printer: press the Go button for a few seconds until the display changes. Or, in your computer's print driver settings, look for an 'Eco' or 'Quiet Mode' checkbox and uncheck it.
Other speed drains: Complex documents (graphics, fonts), network traffic if printing wirelessly, and 2-sided printing. Automatic 2-sided printing runs at up to 15 sides per minute (7.5 sheets per minute), which is half the simplex speed.
For raw speed, use a USB connection, turn off Quiet Mode, and print simplex.