Slow sharing

Dears all

that's the scenario :

1 Mac Pro connected to a Netgear DGN2200v3 (Etnet 1) for internet connection : Etnet 2 connected to a router for internal fast network.

Other 4 Macs connected to the Etnet router.

The problem i get it's : if the Mac 1 is connected to the Netgear i get slow access to the others Macs, if not the sharing ( for example screen sharing ) it's fast as usual. Previously i've used another Netgear wireless Etnet modem and all was fine.

I think there is a need for a internal DGN2200 settings.

Please help.

Thanks in advance.

-A