%USERPROFILE%\Documents\Arma 3 - Other Profiles\<user-id>\Saved\steam
Set Launch Options...
For AMD CPU users:
-noSplash -cpuCount=2 -high
-noSplash -cpuCount=4 -high
-noSplash -cpuCount=6 -high
-noSplash -cpuCount=8 -high
For Intel CPU users:
You can also use --eon_force_display=x command line parameter on linux builds, where x is your monitor ID.
Under steam, set command line like this : %command% --eon_force_display=0
Some servers tend to give users worse FPS. This is because your FPS is tied to the server tickrate, which may give performance hits.
This issue is a long-standing and frequently-reported problem. The developers have yet to comment on this issue on the official bug tracking site.
It is a known issue on Windows 10 Insider builds.
Seems to work for some users, others not.