Are those speeds network speeds or hardware? I suspect they are the latter - given the header of Write Speed and are potentially a limitation of the MacBook. The difference is due to start up and end sequences.
Can you get the Win10 version of task manager running and monitor the network? A GB file should transfer in around 8 seconds whilst the hardware will take 12 seconds to write. Try that and see - and repeat with a 2GB file -16 seconds to transfer and 24 to write. You will also be able to see the data rate (approx) is it close to 1Gbps or 700Mbps?
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