Toisto allows you to practice on your computer so, assuming you have a decent keyboard and good typing skills, you can practice pretty fast.
Toisto knows about language. It has a simple language model so it can quiz you on aspects like grammatical person, mood, gender, verb tenses, diminutives, and degrees of comparison.
Toisto allows practicing from any language to any language that Toisto knows about, not just from and to English.
Toisto is focused on retention. It quizzes you repeatedly on new words. But when you answer quizzes correctly, it quickly starts increasing the time between repetitions, as long as you keep answering correctly.
Toisto is minimalistic, allowing you to focus on the language and practicing. If you need an app to reward you with gems and treasure chests to stay motivated, Toisto may not be for you.
Toisto is open source and free, and will always remain so.
Toisto is extensible. You can add your own concepts.
Add more concepts.
Have Toisto make the user progress through the concepts in a logical order.
Add more languages, but this requires help from native speakers in those languages.
Add more grammar and semantics. For example, more verb tenses.
Add more quiz types, such as cloze deletions or reading exercises.
Explain grammar.
Internationalise the user interface.