API call/endpoint causes insane amount of CPU load

Hi guys!

So my Meteor app also provides its users with an API, which is set up with kahmali/meteor-restivus. It works pretty fine but for some reason, when I do a POST request to my endpoint receiving about 20kB of data (JSON Obj), this API call takes about 30 seconds to complete and uses 90-100% of my CPU!
The weird thing is: I use the exact same function handling the data somewhere else in my app: there I read a .csv file from the client, parse it to a JSON and then do some DB stuff.
When I upload the same data, so about 20kB in a .csv file, the whole call takes about 2-3 seconds.
So I really have no idea why the API call (again - using the same function handling the data behind the scenes) takes so much time and CPU power to complete, while the csv upload does the same thing and is so much quicker.

Any ideas on this?

best, P