PPM values are usually quoted as '... up to ...", as the actual speed will depend on a number of factors, such as:
Paper size
Simplex or duplex
Complexity of page
The last of these (complexity) tends to have the greatest effect on slowing things down.
If your pages contain complex graphics, the rendering engine has to work much harder to rasterise each page, and may be finding it hard to keep up.
What is the speed like if you print (say) 25 pages from a simple NotePad document?