Why does a game lag if 2 .exe are running at the same time.

I'm working on a game mode where we want to have 2 separate executables running of the same game. I have them built to Run In Background. However which ever game I don't have as the focus window seems to run at half speed of the other game. Is there an option I can change to fix this or is it something Unity just does and can't be changed?

This is mainly due to your Operating System allocating the most resources to the active executable. It's something you don't really have control over. I would assume you're testing some kind of networking setup between two instances of the application.

I have found it possible to run multiple Unity builds quite easily, but you must make sure you have at least a dual core processor. It simply won't work properly on a single core machine as the context switches are too expensive for the OS to allocate much CPU time to any background applications. I have found however that running one version in the editor and a separate build seemed to be a little happier than just two builds so could be worth a try.

Overall though, your best bet is to have a second machine running the second copy of the build. I know it isn't always possible, but it is generally best to test in an environment that the app will be used in, rather than the unlikely event of it being run twice on the one machine.

For some reason I have noticed unity lags a lot for certain scenes and I would guess that two unity games running at the same time would dramatically decrease the frame rate. Maybe Unity is like that where it runs one window at full speed and the second at how ever fast the computer can handle it. But again, this is just an assumption. Hope this helps a little.

it's ur pc/mac trying to allocate more power to the window in focus, which will cause the other to slow down dramatically. I'm not sure why you'd want to have two open, it's really not a good idea without heavy optimization and occlusion culling etc.