I finally got an HP CM3530 which is MILES ahead of the Canon machine it replaced.

One feature I like which I didn't think it had is that while it flips the page to print the other side, it starts printing the first side of the next page, making duplex printing much faster than the Canon. However, it doesn't always start printing another page while flipping the previous sheet, why does it do it only sometimes? For example during a print job it might do 20 pages or so printing another as it flips the previous one, then it might do 20 or so while no starting another page while flipping the previous sheet.

Why does it not constantly print in the most efficient duplex method?

Thanks