Dynamic Universal War System (DUWS) seems to suffering really quite badly because of this.
Very easy to test .. fill Altis with enemy zones=20 and max distance=50km.
After setup FPS near 14 (Altis in editor 20 - 40 without AI).
Taking few zones and add FOBs (with extra defense) FPS start to drop down to 7 - 8.
AI stops shooting at some point (rookie level) and also own side AI have difficulties to shoot anything.
Graphics settings not affect to this at all, GPU fan speed seems to slowing so I assume not enough job to GPU.
Of course might be DUWS which does something strange, but I also see bit similar GPU issues on MP co-op and even wasteland.

Quickly tested Arma2 in Domination co-op and I can't see similar issue even when there is quite much AI activity on-going.

Uninstalled ZA and started to work
Before it didn't even create AppData content.
At least two different application error level events (in Event Viewer) before uninstall of ZA.
OS 64bit Win7 SP1

Faulting application name: arma3.exe, version: 0.5.102.571, time stamp: 0x512deb6a
Faulting module name: ntdll.dll, version: 6.1.7601.17725, time stamp: 0x4ec49b8f
Exception code: 0xc0000005
Fault offset: 0x00039363
Faulting process id: 0x1adc
Faulting application start time: 0x01ce19e0b68edb38
Faulting application path: C:\Program Files (x86)\Steam\steamapps\common\Arma 3\arma3.exe
Faulting module path: C:\Windows\SysWOW64\ntdll.dll
Report Id: f51b9fda-85d3-11e2-81b4-005056c00008

https://dl.dropbox.com/u/1402817/arma3_ntdll_module_error_report.wer

and

Faulting application name: arma3.exe, version: 0.5.102.571, time stamp: 0x512deb6a
Faulting module name: unknown, version: 0.0.0.0, time stamp: 0x00000000
Exception code: 0xc0000005
Fault offset: 0x03f831a0
Faulting process id: 0xd04
Faulting application start time: 0x01ce19dfe9309710
Faulting application path: C:\Program Files (x86)\Steam\steamapps\common\Arma 3\arma3.exe
Faulting module path: unknown
Report Id: 280980ff-85d3-11e2-91dd-005056c00008

https://dl.dropbox.com/u/1402817/arma3_nomodule_error_report.wer

Uninstalled ZA and started to work
Before it didn't even create AppData content.
At least two different application error level events (in Event Viewer) before uninstall of ZA.
OS 64bit Win7 SP1

EDIT by millhauz

https://dl.dropbox.com/u/1402817/arma3_nomodule_error_report.wer