Windows 11 systemYou'll have to give more information.
Without more details I might was well take a wild guess at an IP conflict.
- What is the 'correct' speed?
- What speed is it actually running at?
- Have you checked the NIC configuration in Windows (assuming Windows, you haven't mentioned) (IP, subnet, duplex, port speed etc)?
- What do you mean by 'kills wifi'? Wifi on the device, or wifi around the home?