OK, try to eliminate all unnecessary network stuff: Delete all network protocols except IPv4, Uncheck IPv6.
Disable IPv6, LMHOSTS, NetBIOS, can be done via those commands, run CMD as admin and copy/paste:
netsh int ipv6 isatap set state disabled
netsh int teredo set state disabled
netsh interface ipv6 6to4 set state state=disabled undoonstop=disabled
reg add "HKLM\System\CurrentControlSet\Services\Tcpip6\Parameters" /v "DisabledComponents" /t REG_DWORD /d "255" /f
reg add "HKLM\System\CurrentControlSet\Services\NetBT\Parameters" /v "EnableLMHOSTS" /t REG_DWORD /d "0" /f
wmic nicconfig where TcpipNetbiosOptions=0 call SetTcpipNetbios 2
wmic nicconfig where TcpipNetbiosOptions=1 call SetTcpipNetbios 2
You might also use
TCP Optimizer, which polishes some network settings.