I know Gearbox is probably hard at work at optimizing the menu system so its less laggy.
Just figured I would share what happened to my group last night.
We played Cistern of Slaughter with 4 players on Mayhem 2. When we got to the later Rounds/Waves if the Mob of creatures was fairly large, it was virtually impossible to access the menu system. Opening the menu system was probably a good 15-30 second lag before it would fully display. And if you needed to tab over to one of the other menus, forget about it, it would take another 10-20 seconds. We all had to make sure we had the gear we needed before starting the round. If not we had to make sure the menu was left on the backpack to ease swapping out weapons.
I really hope the first major game patch fixes/optimizes the menus.