Problem solved. Open command prompt as administrator and run the following commands:
netsh interface tcp show global
netsh interface tcp set global autotuninglevel=normal
netsh interface tcp show heuristics
netsh interface tcp set heuristics disabled
Do not forget to restart the computer in the end.
The cause was auto tuning was set to disabled for some reason on all my windows 10 PCs.
After restart PC the speed instantly went to 8mb/s