[BUG] - Problem with slow download speed

[mjungnickel18]

Describe the bug
Abysmally bad download data rate of 0.2 mbit/s.
Upload seems fine. 50 mbit.

Smartphone (please complete the following information):

• Device: Samsung S21 Ultra
• Android Version: 14
• App Version 3.6.6
• Backend: Userspace

To Reproduce
Steps to reproduce the behavior:

I exported a config from my fritzbox
I load that config into the normal wireguard app. It works fine, 50mbit/s
I stop wireguard.
Load the same config in wg Tunnel. Activate it. Download is only 0.2 mbit/s

Expected behavior
Same speed for both apps.

Screenshots (Only if necessary)

Additional context
Connection is from Congstar (German mobile provider) to Vodafone Cable Home Internet.

If I test this from the same phone with O2 as mobile provider, both speed tests work the same.

I use an openspeedtest docker container as test subject.

I have no clue with these symptoms....