MyKMU Office L (Fibre) - "Slow" download speeds

Hello

I’m seeing more and more slow download speeds on my MyKMU Office L. Upload speeds are generally very very good (800-900mbit), download speeds are comparably poor (100-500mbit) and vary a lot througout the day.

The Setup:

Centro 2.0 Router, one highly optimized Linux server with a public IP through DMZ, a couple devices behind NAT. With all devices behind the NAT turned off except one and just the Server in the DMZ running, I still only see the mentioned download speeds. Is this a congestion issue in general in the region of Berne? What steps can I take to further test the issue?

Thanks in Advance