Hello and welcome to my guide.

I have a low-end pc and i've spent some time looking around the internet trying to find
tweaks to improve fps in this game.

This guide is the result of my search.
My FPS was much higher after using these tweaks.

Hope it helps you.

First things first, you're gonna want to open up your Advanced Settings

There you will see these options:
-Set the options according to your PC specs.

Texture Detail: Set this to Very High unless your graphics card has less than 1GB RAM
Video Memory: This works best when set to default.
Anisotropic Filtering: Set to Very High, same reason as Texture Detail
Antialiasing: Using this setting will result in major performance hits unless you have a fast CPU/GPU combo, use low if you can as anything higher seems a waste.
ATOC: Disabled for performance, else either all trees or all trees + grass with a good CPU/GPU.

Terrain Detail: Up to your system specs, use as needed but generally either Low or High.
Object Detail: As Terrain Detail except this also influences the distance at which you can spot men, however unless you are zoomed in or using optics they will still be difficult to see.
Shadow Detail: This setting can result in a moderate to major performance hit, use as needed.
HDR Quality: Set to Normal, higher settings are purely cosmetic.
PPAA: Set to FXAA - Low, other settings don't seem better.

Postprocess Effects: Set to Off or Very Low, Low+ enables motion blur of which there is too much for most people.
Interface Size: Normal+ if you want to be able to read the degrees on the compass.
Aspect Ratio: see below
Vsync: Off unless you are experiencing screen tearing or have a very high spec computer that is always 60fps+.

ArmA2.cfg and ArmA2OA.cfg

You can find these files in Documents > ArmA 2

Open the documents using notepad or wordpad.

GPU_MaxFramesAhead=1; This may help smooth your frames and/or increase performance.
GPU_DetectedFramesAhead=0; this is the same as above.
refresh=120; This will make VSync work better if you need it.

Startup Parameters

These are the parameters you enter by right-clicking ArmA 2 in Steam and going to Properties, then select Set Launch Options.

-cpuCount= set this to however many physical CPU's you have either 2 or 4.
-exThreads= use 3 for dual-core and 7 for quad-core and only if you have an Intel CPU.
-high sets ArmA 3 as a high value CPU process.
-maxMem= set this to 2048 or about 1GB (1024) less than your maximum RAM, I am not sure if higher values have an effect.
-noPause Does not pause the ArmA window when alt-tabbed in Fullscreen window mode, good for multi-taskers and streamers.
-noSplash gets rid of the intro stuff at startup.
-world=empty Starts up the game faster as it loads empty level.