I'm using lingvist to study German from English. It seems to me, repeat stack grows too fast. First it seemed ok, but after I had learned about 1500 words, its size became enormous. After being fully exhausted in the evening, I find 300 or more cards there in the morning (just 8 hours later). Besides, when I try to repeat all that words, I usually achieve 90% accuracy. It means, 90% words in that stack are useless--- no repeat is needed--- it's just waste of time. Of course, no algorithm is perfect, but if only 30 out of 300 words are useful, it looks suspicious. Thank you in advance.
That sounds like it's broken for you. I'm at 2800 or so words in the German course, and I have to wait about four days with no reviews before my stack grows to 300 cards.
Is it possible you just started and did 1500 cards really fast or something? If so, that'd be the problem: you aren't supposed to do that many at once. I started back in January and was doing 150 cards a day until my life got too busy and then lowered it to 100 a day.
The way the cards work is the more times you get a card right, the further back it's "re-shuffled" before you see it again. If you get it right once, maybe you'll see it tomorrow. If you get it right a second time in a row, maybe a week later. A third time in a row, maybe a month or two later.
A high success rate in repeats isn't really a sign of the practice being useless. It's good to be reminded of a word just as you were about to forget it, rather than forgetting it, and having to re-learn it. Of course many reviews are actually given too early, but if you consistently get a word right, its interval will increase, and you will see it less often.
Having said that, it's a shame that Lingvist doesn't have an 'easy' button or advanced settings allowing you to tweak intervals like Anki.
I think the problem with spikes in repeats is inherent to any spaced repetition program, and especially one like Lingvist where new cards are added sporadically. I would advise you to just do a fixed number of cards per day, and see if the repeat stack gradually goes down.