Sorry i could see i have not answered some questions from you last comment.
But may i know if you have observed on a webui if the usage goes down if you don't use? -> yes on webui
Have you kept device unattended for an hour and checked later if its still 100% ? -> as this happens only on load i didn´t start a big download for such a long time
Where the data is getting consumed ? -> Speedtest server, tried different ones from different locations and ISPs
What is the device connected to Trb on bridge mode -> Macbook pro
Could you confirm if the device is newly booted up now ? And never used before? -> It was from factory state on yes, purchased as new one, inserted sim and i have just connected my macbook via lan on the device to test it and ran an speedtest. After seeing that i receive 30mbit only (contract is 150mbit max which i receive with mobile phone contract from same ISP on same location), i checked what could be the issue and switched from NAT to Bridge mode, entered APN manually, checked for updates (no new available), did reboots of the device and checked the system load, which was everytime at 98-100% when i started a speedtest. When the speedtest was finished it went down again.
I hope this was more detailed for you :)