Just for context, there is a mountain of research into the more general phenomenon of "desirable difficulties" in education -- practice conditions that make the task harder in a way that improves long-term learning. The increased difficulty is often mistakenly interpreted by learners as indicating that they are not learning as well when in fact the opposite is true. https://en.wikipedia.org/wiki/Desirable_difficulty

Spacing is one example of a desirable difficulty; the spacing effect has been known for over a century and it has been replicated so many times since then that it might as well be a law of physics. https://en.wikipedia.org/wiki/Spacing_effect