Problem printing on Inland cover

Hi,
I have a monochrome laser printer (Canon LBP 2900B). The problem is, that when I take print out on multiple inland letter covers, it takes lot of time between prints. Like, between prints, it takes some 10 seconds, but when a A4 sheet is used, it takes the next paper almost instantly.
I've tried many options in printer settings. Will this be an unsolvable problem or are there any tweaks?

Thanks