Success response for promotional entitlement quite slow

I am using custom promo codes for users of my app. My server checks if the code a user has entered is valid and if so, it sends a request to revenuecat, as stated in the documentation:
 

const options = {
  method: 'POST',
  headers: {accept: 'application/json', 'content-type': 'application/json'}
};

fetch('https://api.revenuecat.com/v1/subscribers/app_user_id/entitlements/entitlement_identifier/promotional', options)
  .then(response => response.json())
  .then(response => console.log(response))
  .catch(err => console.error(err));

My code is waiting for a “Subscriber” response, to check, if the promotional entitlement was successful. This all works fine, however it often takes around 60 seconds until I receive a response, varying between 10 and 60 seconds. Am I doing something wrong in my code, or is this behaviour expected?

Thanks so much already in advance!