It's mostly because of the massive amount of objects being created and not cleaned up, which slows down the PC's after time, but we won't rule out a memory leak either :)

I have also this issue in multiplayer sessions after 1,5 to 2 hours. At the beginning i have with my iMac 21,5 (2011 / Bootcamp Win7 64bit native Res.) 30-40 fps with some tweaks in the config file as well in the steam starting options. After 2 hours i get just around 15-20 fps. A friend has a better pc but gets nearly at the same time this issue ^^. Thats why we make and play self-made Missions ;-)

My RAM usage never climbs above 600MB even with Textures to Very high and AF set to Ultra ...

I usually have the program run with a additional argument to use 8192 (8 Gb ) of ram in addition to 4 cores, process priority high etc. I have 32 gb on hand with 64 going into my new build as I type this.

I reccomend deleting all memory allocators in Arma 3\Dll.
That will force arma 3 to use default operating system's allocatar which is usually better than the a3 default one. Not sure if this is the cause of it, but since i did it i have no more memory leaks.

Please let us know in note section if you are still able to reproduce this issue in latest Development build.

If so, please send us dxdiag and files from this folder: "C:\Users\<Name>\AppData\Local\Arma 3 Alpha\"

Can you pack the files in winrar and upload it somewhere?
When the *.rar will be smaller than 2.097k, you can attach it here. Thank you.

For future reporting please refer to "how to guide" http://feedback.arma3.com/how-to-user.html.html

Closing due to lack of info / response.

If the issue is still reproducable for you, please create a ticket for it.