networking - Dos/ Flood Lag even though Port not Saturated

My GameServers had been under some UDP Floods due to which they generated outputs to the attacker which gave the GameServers some huge lags. Thanks to friends at ServerFault that upon different kind of testing, I was able to successfully block the attack. My question is actually something else but it is important to know how the GameServers reacted to the attack and if the machine kept stable or not:

300kb/s Input would cause GameServer to generate 2mb/s Output. So as
the Input Rate kept increasing, output rate would reach so high that

it would no longer be possible for the GameServer to control it and hence it would give a huge Lag until the attack is stopped. Usually the game server starts to lag when it sends out something greater than 5mb/s and under that is controllable.
Theoretically, I was able to receive a 60mb/s output from my
GameServer on inputting 10mb/s. Its just the way the GameServer works
if not protected.

Now on some of my machines, only the GameServer under attack lagged and although the server was generating 60mb/s output, rest of the gameservers on other ports would run fine without lags on the same machine.

But there was another machine which also runs on a 100 MBPS Network port, even 1 mbps input ( and ZERO output because attack is blocked ) even on an unused port would give a constant yellow line ( on the Lag-o-Meter ) to all the clients on all GameServers indicating lag because that line is actually blue under normal conditions. It would remain the same even on 50mbps or 900mbps input. I tried contacting the host about it because I believe its the way their Network is bridged, but they can't help me about it. Anyone else knowing about such issues because if 900mbps input does not Saturate the port, how can 1mbps input lag the servers although port is not saturated and enough bandwidth is available?

Answer

Okay after some research I'll answer my own question.

The issue was low values for UDP/ TCP buffers. So on increasing the values using sysctl or setting them up in sysctl.conf for each reboot. Things went well. I see on VPS machines, already high values were set in custom OS installs but on stock OS(s) the default values were low.