Quoted laser printer speeds only apply when printing multiple copies of the same page. The first copy can take far longer as the page has to be processed by both the PC and the printer. That processing time depends on many factors, including especially the amount of image data involved.
You can get an idea of the amount of processing involved if you look at the spool file, i.e. the file that is actually sent to the printer. That can be much larger than the actual document.
When printing multiple copies of a document, it should be sent to the printer only once; the printer stores the page image in memory for the subsequent copies. If the spool file for multiple copies is larger than for a single copy, something is wrong, and you need to either select the correct driver setting (e.g. disable collation), reinstall the driver from scratch, or use a different driver.
If using the PCL6 driver, keep the resolution setting low (e.g. 600 dpi) as PCL sends images at printer resolution. The BR-Script (PostScript) driver does not suffer from that. To keep image data as small as possible, do not use images with a resolution of more than 300 dpi as printed. Using 600 dpi images can increase the spool file by a factor of 4 and nobody will be able to see the difference.
Here is one way to check the actual printing speed:
Make 10 photocopies of a single page. Start timing from the moment the printer picks the first sheet from the paper tray. The 10 copies should be delivered at spec speed (about 20 sec at 32 ppm).
Best Answer
I finally found that you had to manually tell the driver about the presence of the duplex unit. This is done through Control Panel > Printers & Devices > right-click your printer > Printer Properties.
For the standard PCL Driver...
For the PS Driver...