I just bought the 2/3 days and tried around 10 servers with wireguard and the best speed that I got was 500Mbit/s which is half of my speed.

Also, most of them did not even pass 100Mbit/s.

Am I doing something wrong? I had great speeds with mullvad with the respective set-up.

The noteless app has not been updated since 2021, you should replace it. If you need something minimal check out notally, if not there are other feature rich apps that are actively maintained.

Try voyager, last time I checked, eternity was not maintained.