After a recent CPU upgrade to an I9 9900KS, I've noticed that my FPS in Subnautica were actually lower than before, with nothing else being changed. So I've started to investigate the cause and found out that having more than 4 CPU cores/threads available is having a negative impact on performance in the game.
This is most notably around large wrecks and especially in the Kelp Forest biomes, but applies in general. Here is a fps comparison between 16 threads and 4:
As you can see my fps almost doubled after limiting the game to only use 4 threads, which is a massive impact. In general the more cores/threads your CPU has, the worse your fps will be, this goes for AMD as well as Intel CPUs as I've had multiple people over on the Subnautica modding Discord test this on both AMD and Intel CPUs and they all confirmed my findings. It has been tested on an Intel I7 6700k and an AMD Ryzen 3600 among others, it isn't limited to recent CPUs or large numbers of cores/threads, but the more cores/threads the bigger the difference.
It also doesn't matter if your game is modded or not, I've tested without mods and the results were the same, though some mods can certainly impact fps, but limiting the thread count to 4 always improves fps, modded or not.
As for fixing this issue, the easiest way to do this is to manually assign the number of CPU threads the game is allowed to use. To do this all you need to do is open up your task manager, go to the "Details" tab and find the Subnautica.exe, right-click it and select "Set Affinity" then untick the box that says "<All Processors>" and manually tick CPU 0,2,4 and 6 so it looks
It is important to select CPUs 0,2,4 and 6 for people who have CPUs that make use of Hyperthreading, such as the Intel I7 and I9 CPUs or SMT for AMD CPUs, such as most Ryzen CPUs, since those are the "real / physical" cores and 1,3,5,7 etc. are the second logical cores, which will improve your fps to a lesser degree. I've tested this and had others test it as well, this is not speculation, but empirically tested, I'll gladly provide screenshots if needed.
If your CPU doesn't support Hyperthreading or SMT it only matters that you limit the number of CPUs to 4, but you are free to pick which 4 to use.
It is also important to note that setting the affinity via Task Manager has to be done each time the game is started, as Windows doesn't remember your settings, so keep that in mind, as I haven't found a working command line that automatically limits the game to 4 cores.
If you want to test this yourself, here is how you can test it in real time.
First of all you need an ingame FPS counter, such as Fraps, the one included with Steam or whatever else you use to display your current fps. Then you need to set your game to run in borderless fullscreen and set your Task Manager to always be on top, you can do this by opening it up and clicking on the options menu at the top and select "always on top".
Now boot up the game, go to an area such as the Kelp Forest or close to a large Wreck, preferably in a Seamoth so you don't have to worry about Oxygen, and remain still and tab out of the game and open up your Task Manager and tab back into the game. You will notice that the Task Manager will remain visible despite the game being in focus, this is the "Always on top" setting.
Now you can alt-tab to the task manager, without moving around in the game and start reducing the number of CPUs to 4 and as soon as you click OK in the "Set Affinity" window, you should see an increase in fps. You can play around with the number of CPUs and which ones you assign to see which gains you the most fps, but as I've said above, for CPUs with Hyperthreading/SMT it should only be CPUs with even numbers, for CPUs without it doesn't matter.
I personally noticed a small gain when limiting the game to 3 cores, but I also have a very powerful CPU with a very high clockspeed, so that might not be the case for everyone, just test it for yourself to see whats better 🙂
I hope this helps some people who are having performance issues with the game, despite, or in fact, BECAUSE, they have a powerful gaming rig.
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© Post "[No Spoilers] Warning, more than 4 CPU cores/threads has a negative performance impact on Subnautica. + Fix." for game Subnautica.
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