Get Options via API causes Timeout-Error

My blueprint looks like this:

    label: Code-Element
    type: select
    options: api
      url: "{{ site.url }}/mm-api"
      text: "{{ item.title }}"
      value: "{{ item.value }}"

This is my url:

And here is what i get if i open this url in my browser:

If i do this, the panel loads really slow and the field comes up with an error:

Operation timed out after 10004 milliseconds with 0 bytes received in file: /path_to_my_project/kirby/src/Http/Remote.php line: 288

This should actually work and I would only expect a timeout with a lot of data. What is in your route?