I rode in kei-trucks and kei-vans with an adult in every seat, up some very steep and long hills in Japan. We weren't exactly in the fast lane, but there was not any mention of the van not being up to the task. It felt just fine.

In fact my friend was SO excited about his new van (for 1998) that for a while, he was bragging about every last little aspect up and down those hills, every time we rode with him. He lived in a gigantic danchi at the top of one of the steepest hills around, and still loved that thing.

(He worked as a furnace-jumper-inner / furnace-insides-scraper at an auto recycling place all day, and had molten aluminum burns on his arms and sometimes face...I respected the heck out of that guy for what he did to support his family. I think he runs an IT business in Brazil now.)

Those things are used in mountains in Japan without any issues at all, they are very light.

Japan is famously mountainous. I can’t imagine they don’t road test them on “real hills”.

Our family car is a kei minivan (microvan?) with power sliding doors (=heavy) and everything.

No trouble going up long or steep hills in windy mountain roads with the whole family of 4+luggage in the car.

No trouble going up long hills at highway speed (120 km/h) with two adult friends in the back.

Aircon on and everything. Works great.

Why do you believe that?

Gear ratios, you might just not be getting up the hills very quickly