Most flashcard apps ask you to build your deck the same way: click "new card," type the front, type the back, click "new card" again. Do that thirty times in a row for a single vocabulary list and you'll understand why so many decks in so many apps stop at twelve cards and never grow again.
The friction isn't reviewing. Review is the easy part - a few minutes, a few taps. The friction is everything that has to happen before the first review: turning a list you already have into thirty individual card-creation actions.
Where people actually quit
Nobody quits spaced repetition because reviewing is hard. They quit because making the cards felt like a second job layered on top of the thing they were actually trying to learn - a vocabulary list, a set of definitions, a page of terms from a textbook. If getting that list into the app takes longer than the studying itself, the list stays in a notes app instead, and the deck never gets built.
Paste the list, skip the clicking
Cirello supports bulk import: paste a list of front::back pairs and the whole set becomes cards in one pass, instead of one "new card" click per pair. A vocabulary list you already typed out, a table of terms and definitions, a set of dates and events - if you can format it as pairs, you can drop the entire thing in at once.
The list you already have is the deck. You shouldn't have to rebuild it one click at a time.
This matters most for exactly the content that's easiest to procrastinate on adding: the long list. A three-card deck is no trouble to build by hand. A hundred-card vocabulary list from a language course is where one-at-a-time creation quietly loses.
Duplicate detection, so pasting twice isn't a problem
Bulk-pasting a list you've added to before raises an obvious question: what happens to the cards you already have? Cirello checks for duplicates during import, so re-pasting an updated version of a list - or importing an overlapping set from two different sources - doesn't silently double your deck. You get the new cards without cleaning up copies of the old ones afterward.
That removes the other reason people avoid bulk import even when they know it exists: the fear that doing it wrong means a mess to untangle later.
After the import
Bulk import gets the cards in - it doesn't decide where they live. You still organize your library into folders the way that makes sense to you: by course, by book, by language, however you'd sort it if you were doing this by hand. The import just removes the one-card-at-a-time bottleneck between "I have a list" and "this list is in rotation," so the organizing is the only manual step left.
One caution: because pasting is easy, it is tempting to add far more than you can review. Every card you add returns many times over the following weeks, so how many flashcards to review per day is worth a read before you import a big list, and how to write good flashcards shows how to shape the pairs so each card asks for one thing. For vocabulary lists specifically, see how to memorize vocabulary in a new language and the page on language learning. The overview of the whole app is on the flashcard app page.