Spaced repetition happens everyday outside of a "sit at a desk and learn" setup and is plenty efficient without effort and pain. The obvious one is seeing again and again a concept so much you end up remembering it, like the Dunning Kreuger effect (or to take it darker, being able to recipe an ad you've seen hundreds of time)
If anything, I'd argue the pain and effort can be negative it will still work.
Some things you learn without really trying. We learn our native language by repetition because we hear it all around us. But the question here is what if you want to or need to learn something you won't learn automatically? What helps? Try spaced repetition. It doesn't happen every day automatically. You must force yourself to do it even though it is tedious, if you want to learn what you want to learn.
I think what GP meant is that we already learn a lot of things through "spaced repetition" even if we're not consciously applying that technique. E.g. if you drive the same route repeatedly, eventually you'll remember the way without needing to look at a map. That's spaced repetition.
If anything, I'd argue the pain and effort can be negative it will still work.