I’m assuming that this is an external API that you’re calling over HTTP, or something like that?
How are you calling this API, are you using a library?
Does your custom action start with async def, and does the call to the API have an await in it? If not, then you might be making a blocking call to the API, which means that while you’re calling the API you’re blocking the rasa actions event loop, and so it cannot process any other requests until that call is finished, it just has to wait around and be blocked.