Doesn't matter which game it is, it seems like using the most basic lighting setting, Static Lighting, is the only one that results in a decent framerate. If I set my lighting on anything above that the framerate takes a serious nosedive in all three games and stuttering becomes worse (I already expect occasional split second stutter every now and then because of the game loading up assets as I move, but with any lighting setting above Static Lighting it gets way worse).

I'm playing at 1600x900 with a GTX 660 and AMD FX-6120, so sure while my PC isn't top-of-the-line for 2016, it should be able to run a bunch of late-00s games no problem (heck I know that for a fact, I own a decent number of games released in the late 00s that run very smoothly). I should also note I'm playing vanilla, no mods are being used at all (so as such if any mods or fan patches can fix this issue then I'd like to know which ones).

I'd just like to be able to play with the highest lighting settings with a decent framerate. Is there any way to do that, or am I just going to have to bite the bullet and settle for playing the entire series with Static Lighting only?

(I also attempted to check my CPU affinity because I remember one other game I played in the past had a terrible framerate for a game from the mid-00s, until I found out the CPU affinity for some reason defaulted to only one of my six cores, and enabling all cores fixed the problem instantly, but when I try and set the affinity for any STALKER games I get an error message saying I do not have permission)

The Xray engine that stalker uses only supports 1 core so maybe it is a CPU bottleneck, but I played it fine with dynamic lighting and improved graphics mods on a PC that was much older and worse than yours with absolutely no problem.
Have you tried the float32 mod? It tends to improve performance for most people.
There's also the zone reclamation project that fixes most of the bugs without modifying the gameplay, that might help with your performance issues.
Most importantly make sure global illumination is off, it's in the .ini found in the same folder as your saves and the engine totally shits the bed if it's enabled.

Most likely scenario is the bottleneck is just too great. My CPU has six cores, and I have no idea what speed only a single core runs at.

I checked to see if performance would differ with ZRP and sadly it doesn't. Full dynamic lighting even with ZRP still gives me the same choppy framerate and those split second jitters that sometimes happen when running only get worse with full dynamic lighting turned on (instead turning into a couple of seconds of the framerate dipping into what feels like sub-30). The absolutely worst jitteriness is always near fire for some reason. Like the fire inside the tunnel at the start of SoC or the fire near... Nimble I think his name is (that guy you save during the very first mission). Stand near the fire and suddenly it's like the game has a framerate similar to how dolls and figures in Robot Chicken move. Oh and also global illumination is off, apparently it defaults to off anyway.

I did a quick check on Clear Sky as well and yeah when I turn on DirectX10 the framerate REALLY goes to crap. Like when I look towards the bar, campfire, etc, the framerate goes sub-30, and the jitteriness or "lag spikes" are REALLY prevalent. Those same lag spikes are still there even when I change to DirectX9, even if the framerate is better.

Honestly the lag spikes might be my biggest problem. I can tolerate a framerate that's between 30 and 60 so long as it doesn't suffer from so many lag spikes that make the game feel like it's momentarily freezing up every 5 seconds. And yeah again that's my biggest issue here, only DirectX8 provides me with an experience where the lag spikes or "jitters" are kept to a minimum I can tolerate. DirectX9 and above always results in lag spikes that make the game feel damn near unplayable because I can't stand to see the games lag spike like this every 5 or 10 seconds, or go into sub-30 territory when I'm around fire or inside some buildings.

What I really wish I had is a way to force the games to use all six of my CPU cores because then the games wouldn't be bottlenecking this badly, but again trying to alter the CPU affinity via task manager doesn't work, I get an error message saying I'm not allowed to do that.

New update on my situation as it looks like I've found the best compromise that I can. The only way I can fix the lag spikes appears to be playing the games in windowed mode.

In SoC: those annoying lag spikes or jitters I got around things like fires? The moment I switch from fullscreen to windowed the problem went away immediately. Kinda funny how much a simple thing proved to be the best fix.

Now in terms of better fixing performance (admittedly more so for Clear Sky and Call of Pripyat) I've opted to "bite the bullet" in another way. I'm now playing windowed at 1280x720 resolution (using the -center_window command line switch I got from TweakGuides so the windows aren't stuck to the top left corner [aside: I hate it when windowed mode in most games defaults to the top left corner, but at least most windows can be moved, though annoyingly some games have unmovable windows, and the entire STALKER series is technically like that but thankfully the command line switch helps greatly]). Yeah I know for some having to settle for your non-native resolution can seem "immersion breaking" but eh I'm fine with it. I've had to use windowed in some other games in order to get better performance, like Far Cry 4, as fullscreening that game gave me sub-30 FPS.

So yeah safe to say I've basically found the "fix" I was looking for.

I played stalker with dx9 on a Phenom 2 X3 which was at 2.8ghz, google tells me the FX-6120 should have 3.5ghz single core performance and a 660 is more than powerful enough so I really have no idea what would be causing this other than just some weird quirk of the Xray engine that just hates your configuration or something. The performance around fire and inside really seems like it might be global illumination but yeah, that's disabled by default.

Anyway it's good that you managed to get it working, once you're done playing it vanilla I recommend checking out the AMK mod, it adds a lot to the game without changing the graphics so it shouldn't have too much of a performance hit.
The momentary freezing is a quirk of the engine when it loads stuff, even on my current PC I get a short freeze when approaching the bar as it loads a crapload of NPCs.

Hey,

A bit late to the party.
I know you found a compromise, but please try my solution bellow, and tell me if this helps.

Open nVidia control panel (should be a shortcut by right-clicking on desktop), select the desired STALKER game and apply these settings:

- FXAA: On
- Maximum pre-rendered frames: 1
- Multi-display/mixed GPU acceleration: Single display performance mode .
- Threaded optimization: Off
- Triple buffering: On
- Vertical sync: Adaptive

Apply all these, then set all settings to desired values in game. The stuttering - especially while looking at fires - should be dramatically reduced (almost non-existent).

Please do tell if this hels.

Sorry to say, it didn't really work. It seems the only way I can fix this oddity is I have to play in windowed mode, as full screening is what's causing the issue in the first place.

I am sorry to hear it didn't work out for you.
However I think this may be related to the fact that your CPU is AMD-made (I am not criticizing the brand) and if I recall correctly, I might have read somewhere that X-Ray engine is optimized for Intel architecture.

Let's try something else:

1. Right click on game .exe file - Properties - Target and add "-noprefetch" argument.
It should look something like this: *\game_install_folder\bin\XR_3DA.exe" -noprefetch
2. Install this small patch so that game EXE uses more than 4GB ram.
3. Set your CPU affinity to STALKER to Priority HIGH (after you start the game, tab out, open Task Manager, locate the game and right click).

Do tell if this helps.