We have a cargo bike, which is a 1990s hard tail mountain bike with an Xtracycle extension. It weighs roughly 35 lbs. When I take it to the grocery store and fill the panniers, it holds about four sacks of groceries and a couple gallons of milk, call it 50 lbs of stuff.

The weight makes a huge difference. Even on flat ground. The bike handles more slowly, is harder to balance at a stop and at slow speeds, you accelerate from stops more slowly, and braking is compromised. There is no way you could fail to notice the load on the bike.

When I come to a hill, the extra 50 lbs puts me a full chainring lower.

What I am saying is, I am surprised that the OP doesn't notice any difference from a 30 lbs load on his bike. Either the load isn't really 30 lbs, or he and his bike together weigh a very large amount so that 30 lbs is a minor percentage increase, or he is riding slowly in a low gear on fairly flat ground, or he is quite insensitive.