It does have some ux benefits too. The "back gesture" works much better than on other browsers. Firefox gives no feedback, and chrome gives very poor feedback.
Safari lets you peek at the previous page, and makes it very obvious that you are about to go back, without any performance hit.
It also comes with reading mode, which chrome doesn't. Even with an ad blocker, many pages are still layed out in ways that can only be summed up as reader hostile.
So for reading and moving between pages, I feel like it beats both ffx and chrome. So I disagree with your "more important than anything else" part, but agree that performance and battery is among its top features, and if you don't care for that, safari does become a hard choice.