An internet speed test from a PC is a quick way to check everything is fine but not good for fault finding. A test to router (so no LAN/WIFI/PC involved) would point to a ISP Router/ONT/OR issue such as a speed upgrade not happening - but as you've already moved on to testing data transfer device to device on the LAN it's best to be looking at LAN speed test apps and not internet speed test results.
The main requirements for W11 are a compatible processor - a 6700K or older isn't OK for example, a GPT format disk and CSM off for UEFI boot, and TPM - just a BIOS setting such as PTT for Intel. I wouldn't recommend work-arounds as one day they will cause a compatibility problem - but not something like a low transfer rate.
As before, after checking for any unnecessary background apps, and assuming the LAN driver is up to date and at default settings, and the cable and both the PC and LAN ports arent broken, I'd normally just do an in-place re-install (the fix problems with Windows Update option in recovery settings) - it's fairly quick and brings things back as they were - but I'm not sure what happens if the PC is not W11 compliant.
