Most apps that say 'spaced repetition' don't do it
The phrase has been diluted to the point of meaninglessness. A very large number of study apps implement "spaced repetition" as a fixed ladder — show it again in one day, then three, then a week — regardless of whether you found the card trivial or impossible. That is a reminder schedule. It is not spaced repetition, and it wastes most of the review time it takes from you.
Real spaced repetition models *your* memory of *this* card and picks the moment to review it: late enough that recalling it is effortful, which is what strengthens the memory, but early enough that you don't fail it outright. That moment is different for every card and every person, and it can only be computed from your review history.
What FSRS actually does
FSRS keeps a memory state per card — how stable the memory is, how difficult you personally find the material, how retrievable it is right now — and updates it every time you rate a review. The next interval is chosen so that your probability of recall decays to a target when the card comes back. Because the forgetting curve is modelled as a power law rather than an exponential, the long intervals it produces are much closer to what actually happens in human memory than the older SM-2 family predicted.
The result, in practice: fewer reviews for the same retention, and the reviews you do get land on the cards that need them. We have written the comparison out in full in FSRS vs SM-2, and the underlying theory in spaced repetition explained.
The part that is genuinely different
Anki has excellent FSRS. What Anki does not have is your lecture slides. The cost of spaced repetition has never really been the reviews — it is making the cards, which is slow, which is why most people who buy into the idea quit in week three with forty cards and a bad conscience.
Thaxas makes the deck from your material and then schedules it properly, which closes the gap between the two halves of the problem. If you already have the discipline to hand-write cards, Anki is free and superb, and we say so.