Slow data transfare rate on WDEX2 on Linux

Hi,
My WDEX2 is connected using 1GB/s to LAN. Also my Linux machine, ubuntu 18, also 1GB/s connection.
For some reason data transfer rate which was roughly ~ 40MB/s (when Windows was installed on same PC ) is now 4MB/s when Linux is now installed( transferring 2-6 GB files ).

What can be the reason for that ? I’m not sure it has something to do with OS/

Guy