I am sure this has popped up many times and I realize this is an Alpha but I'd still ike to contribute to the (probably) massive pool of performance issues reports.

I am running the following:

AMD Phenom 9650 x4 @2.3 Ghz
GTX 470 (1280 MB GDDR5)
8 GB RAM
Windows 7 64-bit

That's about the important stuff I suppose.
I play on Standard Quality, with fancy stuff such as AA, Anisotropic Filtering, PP all disabled. Here, it's pretty obvious that Standard Quality looks worse, but increases performance. That wasn't the case in ArmA2, so good work on that.
But I don't know what's with my computer.

Just to give you an idea, I'll compare my usual ArmA2 performance with ArmA3.

ArmA2 runs on 50-60 FPS solid with mentioned rig, both in SP, but especially in MP as the CPU has less workload. Chernarus gives me a performance of 40-50 FPS solid due to massive amounts of trees.
ArmA3 runs on 15-20 FPS (where 20 FPS is max) when I am close to towns or if I spawn atleast 1 whole Squad. The same applies to underwater. Workload in MP is reduced so I get 30 FPS when playing with 15-20 people.

If I am all alone, I get 40 FPS max in ArmA3. In quiet areas. Forests are a different thing, where I definitely lose atleast 5-10 FPS.
In ArmA2 I run at roughly 70 FPS when alone (I don't cap my FPS so yeah)

Some people will probably start wondering "OH MY GOD HE MUST BE LYING", yeah well no. I have managed to configure ArmA2 well enough (as ArmA2 is not obvious when it comes to which option puts more workload on what. Shadows for example. Put 'em on low and you put workload on the CPU. Do the opposite (on high) and you put workload on the GPU).
ArmA3 definitely went a step forward in properly configuring things (again, putting everything on low helps a bit), but it seems to me things are still CPU-heavy.

Now you'll probably also wonder how this affects things. Besides the obvious pain playing with constant 15-20 FPS, it also slows down reaction time. AI Movement, AI Reaction time, Game Reaction time. EVERYTHING is chewed through by the CPU. So since my CPU is slow as fuck, and combined with the general AI failure, it usually takes the AI atleast 3 seconds before they notice I'm there. I can engage them freely without them doing anything.
Opening a door takes ATLEAST 6 seconds after performing the action before it opens. Never less, always 6 secs or more.
Please don't tell me to "buy a new CPU". I realize my CPU is shit but this can definitely be improved from ArmA3's development side. If ArmA2 runs better than ArmA3 then there's really room for improvement in A3.