Yes, I agree, that is what they meant by slow animations. So why did you bring up compute power? They specifically said "not because poor performance".

Also I don't think "just" is the word to use here. Slow is slow, and when it's on purpose it's harder to avoid.

Slow is in the eye of the beholder. Of all the legitimate complaints one could make about Apple, “slow” is somewhere towards the bottom of the list.

The threshold of what is too slow is in the eye of the beholder, but there have been good studies done on that topic and small delays do cause problems.

But the general idea of things being slowed down by animations is objective. It could be done in a frame or two, it takes X frames. And you can add up those delays when you're navigating and reach significant numbers.