Throughput between MacOS and Windows devices limited to 100Mbps, how do I remove the bottleneck?

I am using an M1 MacBook Air on the latest OS Update and a Windows 11 desktop PC and I would like to move files between them in the realm of a couple of GiB (1-100, so transfer speeds do matter). They are connected in the same Network via either ethernet Cat6 or Wifi 5GHz (for testing and stability I used ethernet, but in theory 5GHz Wifi would be plenty). The network includes a router and the Windows machines are connected via one dumb ethernet switch (i.e. non-configurable), all 1000Base-T, machines verified to be connected and recognise that. The issue: Throughput between MacOS and Windows is limited to 100Mbps. I can't figure out where the bottleneck is and how to resolve this. Multiple restarts etc. didn't yield any results, different programs and transfer protocols didn't either. I used iperf on all machines - MacOS and Windows have a throughput of around 96Mbps and Windows to Windows the expected 980Mbps. Any tips, something I overlooked? A setting? Maybe someone has a recommendation for a (free) software solution to easily share files between the systems?