Set Depth of Field option to low without significant visual impact.
Additionally you can turn off Ambient Occlusion and FXAA which affect the speed when aiming down sights.
If you're experiencing mouse lag from vertical sync and screen tearing without it, selecting Borderless Windowed mode from the video options menu may solve the issue.
"Error during Intialization:
ERROR: sound Bank failed to load cmn_root.Englisch. You have a Build Problem."
Set the language of all three game parts (singleplayer, multiplayer and zombies) to your desired language and wait for Steam to finish downloading.
Delete friends list until total is under 60, or add more friends until total is over 100.
Set Steam in Offline Mode. You may not unlock achievements.
You can block "t6sp.exe" for the campaign and/or "t6zm" in your Firewall by creating a blocking rule for incoming and outgoing traffic. Achievements works, but you won't be able to play online or retrieve leaderboards.