I also am experiencing this same issue. The new M601 is printing 1 envelope every 10 seconds. My HP4250 does 1 every 5 seconds. I have tried both PS and PCL6 drivers and neither speeds up the printer.
Watching the printer during the actual print shows the following:
Pick envelope from envelope feeder.
Print envelope
Eject envelope to paper exit
Pause 5 seconds
Pick next envelope.
The timing is that accurate.
I printed 30 NON-Collated envelopes from Microsoft Word to the paused printer.
I enabled the printer and started the stop watch at the same time.
If I print the same envelopes using Tray 1 (feeder removed) there is only a 2 second pause between envelopes.
I certainly hope this is an internal setting that can be changed as the users are ready to return this new printer and return to their prior 4250 due to the amount of envelopes they print.