How to keep a game running when tabbed out

Click away and the game freezes, drops to a few frames per second, or stops simulating until you come back. Three different things cause it: the game's engine, Windows, and sometimes the graphics driver.

How to keep a game running in the background

Second Screen Gaming offers the fixes that fit the game:

Keep the game running while unfocused, the focus assist, sends the game the activation messages Windows sends when you really switch back to it, once a second while it is in the background, so engines that listen for them keep running. Answer the instant focus is lost makes it reply straight away.
Unreal Engine games ignore those messages and ask Windows which window is in front, so they get a configuration fix in Engine.ini instead. Start the game once before turning it on. Games that cap their background frame rate in their own code are not affected.
Source games get engine_no_focus_sleep 0 written into autoexec.cfg, so they stop sleeping between frames.
Stop Windows from throttling the game clears the background power throttling behind Task Manager's Efficiency mode for the game's process.

The tray agent re-applies the runtime assists every time the game starts, so there is nothing to redo. The focus assist and the throttle exemption are switched off for games where anti-cheat is detected.

Coming soon on Steam

Why games slow down or stop in the background

The engine. Many engines stop or slow their game loop while the window is inactive. Unity games stop entirely unless the developer turned on Run In Background, Source games deliberately sleep between frames, and Unreal games can idle when they are not in front.
Windows. It can throttle programs running in the background to save power. Task Manager shows this as Efficiency mode.
The graphics driver. NVIDIA's Background Application Max Frame Rate setting, when turned on, caps any game that is not in front.

How to keep a game running by hand

Unity games cannot be changed from outside, and Efficiency mode has to be checked again every time the game starts. Where these apply:

Look in the game's settings for "Pause when unfocused", "Pause in background" or "Run in background".
Source and Source 2 games: enter engine_no_focus_sleep 0 in the console, or add that line to autoexec.cfg.
Unreal Engine games: in the game's Engine.ini under %LOCALAPPDATA%, add:
```
[SystemSettings]
t.IdleWhenNotForeground=0
```
For Windows throttling, find the game on Task Manager's Processes tab and make sure Efficiency mode is off for it. Setting Power mode to Best performance in Settings, System, Power also helps.
NVIDIA graphics: in NVIDIA Control Panel, open Manage 3D settings and set Background Application Max Frame Rate to Off.

Common questions

Why does my FPS drop when I tab out?

Usually one of three limits: the engine slowing its own loop, Windows throttling the background process, or a driver cap such as NVIDIA's Background Application Max Frame Rate. The first two are covered above. The driver cap can only be changed in the driver's own settings.

Does a game running in the background still use my GPU?

Yes, as much as it would in front. That is the point of the fix, but the game keeps drawing power and heating the PC while you do something else.