The off-hand attack uses your bonus action, and you can toggle dual wield on and off with a switch down below your fixed action bars (underneath the ranged-melee switch; it's grey and very easy to miss). If dual-wield is switched on ,and you're dual-wielding, then your character will automatically use their bonus action to stab with the off-hand weapon when you take an attack action with your main hand weapon. If it's not switched on, then you have to manually choose to use your bonus action to make an off-hand attack each time.
This is a bit clunky right now, and also has a variety of bugs in it that may cause it not to work properly, or to over-perform in breakable ways.
My load times are not that long, but I've heard others make similar complaints about loading, and I think it's Larian's current statements that load time should improve as the game moves towards a finished state.