Is it possible to create multiple Trello cards using only one API call? I'm looking for some kind of batch request.
There is a https://trello.com/docs/api/batch/index.html but it has a parameter: list of API v1 GET routes. I use a POST request to create a card.
I currently use POST /1/cards from https://trello.com/docs/api/card/index.html in a loop, but it's extremely slow.
No, there's no way to create multiple cards through a single API call. You should be able to make multiple calls in parallel (although keep in mind that you will be rate-limited if you abuse this).