The performance was never the consideration for Nintendo. They want a handheld that can last a long time, so they will always clock their chips down.
I fully agree with the first sentence, but i don’t think the second quite hits the mark. The real reason is simply cost.
If Nintendo was concerned with battery life, then they’d still go with a modern processor, but as you say clock it down to hit the efficiency sweet spot over chasing performance. But instead they usually choose something that is already dated at release (even accounting for development time), as opposed to a company like Apple that pays a premium to get first dibs on any new processing node.