I'm experiencing the same issue... After an hour of multiplayer gameplay I notice a gradual decay in FPS....
Can you please explain in more detail how you change the memory allocators for Arma 3?
Thanks
EDIT:
I found this in the readme file:
"The memory allocator is a very important component, which significantly affects both performance an stability of the game."
"If no allocator dll is found, functions _aligned_malloc / _aligned_free (using Windows Heap functions) are used as a fallback. Important note: the Windows 7 allocator seems to be quite good, and it may therefore make sense for some users to delete all custom allocators on Windows 7 or newer)."