I've set up a server (through arma3 -server) and with my DM scenarios available at ArmaHolic ( Its the TDNL pack ). But after 15 to 20 minutes of gamplay the server FPS just plunges below 10 FPS, no matter the hardware I throw at it.
Coming from Arma2 I can't imagine why 20 vs 20 infantry would so heavy. Since there is no AI and little to no logic involved.