Why are new printers so slow in printing a run of envelopes? Are there any fixes?

Neither are those printers a suitable for large quantities of envelopes. Your basically using the wrong black and white printer. Your boss needs to invest in the next up the line He just cheaped out on the Black & white and your paying the price now. Using them in the application you've decribed, you'll soon be really frustrated and having a tech out to repair the manual tray assemblies or replacing them again. Manual trays are meant for OCCASSIONAL convenience envelope printing, not stacks of 100. You need a LaserJet printer that will take an envelope feeder for that kind of volume.

I saw the comment above and she may work for a copier dealership but its probably in the office and not selling, repairing or supporting the machines. Example: Duplicators haven't been made in years. They are called printers, copiers and multifunction machines now. She is also off base with HP sucks. Funny thing how 65-70% of the printers sold on the planet are branded HP. Hmmm... guess the rest of the world is wrong! Trust me, you just need the proper machine for the job, HP or otherwise. Your looking for a laser printer with an envelope feeder. Take a look at the HP 4250 with an envelope feeder attachment. The printer is $899 and the optional power envelope feeder is $299 additional. Sure, I understand it's more money than your boss was intending to spend on a black and white printer but, factor in constantly replacing the manual feed convience trays and rollers and having you stand there feeding envelopes and it quickly becomes cheap. There's a link below.

Good luck.

Let me tell you-those HP printers are good for printing a FEW envelopes at a time, but if you have a lot of envelopes that need to be run (like 2k or more) I would get a duplicating machine.