I've been wrestling with a particularly weird problem for quite some time now. I posted about it in the official Keen help site, but nothing has come of it so far. Yes this is a long post. It's a weird issue and I've been trying for a long time to work it out. To be specific, after some time playing or in certain seemingly random points, it starts slowing down badly. It can feel like it's internally frameskipping a lot in most cases but then I look at the actual framerate and it's locked at a full speed (120 FPS seems to be this game's max?) That's hard to define but it's like it's skipping frames even though it's not. It can feel like it's internally missing frames and running at the next half point down. For example, it can feel like it's running 30 or even sometimes 15 in the absolute worst cases. (I don't see 15 on this system, but I did on the previous.)

But sometimes now it actually literally slows down. As in moving in slow motion. Usually if I press shift+F11 (I wish I had known about that sooner when I first asked about this over there) I see "UPS" has dropped to 50 or rarely even lower when these issues crop up. This actual slow motion movement thing is actually new -- it definitely didn't do that before.

I should also add that this issue occurs on both Linux and Windows. It may be worse in Linux -- I'm not sure since it's so random -- but it happens in both. I will, however, only be running Linux from here on (not optional -- I do not have a license on this system and will not be buying one.) I know Keen does not support Linux, but again, it happened in Windows on two computers.

Someone said the Keen forum that it may be an issue with the game's support (or perhaps lack thereof) for AMD processors, though honestly I've never heard of a modern era game using code that actually breaks on one of the two. It would have to be using something really weird if so. But then it didn't do it on the 2600 back then. Unless it's something more recently introduced. I would assume by now other AMD users would have surely experienced issues if so though. I'm honestly out of ideas. I've tweaked everything I could and no effect. That includes lowering all the settings with no effect (but then FPS is fine, so that's no surprise.)

I also tried running the game separately -- playing on my LAN with the dedicated server on another computer. It still does the slowdowns on the client side (but I don't think the server side ever did, which is interesting because it is my previous computer.)

This issue occurs whether or not I have any mods loaded.

If I exit back to the main menu and reload, usually the slowdown stops for a while, but I think it happens again sooner each time. Exiting the game entirely seems to buy more time before it happens.

My current exact spects:
Ryzen 9 9900X (12 core, 5.65/5.4GHz boost)
Radeon 7800XT (16GB VRAM, no OC)
64GiB (2x32) of DDR5-5600 RAM using EXPOII to match timings (no OC)
Asus B650-Plus Gaming MB
Western Digital Blue SN5000 NVMe

Previous system:
Ryzen 5 5600X (6 core, 4.65GHz boost, but later fixed at 4.4GHz exactly.)
Geforce 4060Ti (8GB VRAM, stock OC,) then later the Radeon 7800XT
32GiB (2x16) of DDR4-4000 RAM
Asus ROG Strix B550-A Gaming MB
Same Western Digital SN5000

I think the 2600 was:
Ryzen 5 2600 (not X) at a fixed 2.8GHz all cores
Geforce 1060 (no OC)
32GiB (2x16) of DDR4 (I think 3200?)
(I forget which MB I had for this one. MSI AM4 200 series I think?)
PNY CS1311 960GB SATA SSD

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As a bit of background and hardware details, when I first started playing this game I had a Ryzen 5 2600 (non-X) and the game mostly ran fine. A few FPS drops here and there, but nothing like this particular issue. I had the 2600 running a fixed speed all cores (2.8GHz if I recall -- Zen/Zen+ only ran one core at full boost and the rest stayed really slow -- 2.2 I think -- so an all core speed of 2.8 was actually a lot) and as far as I know the CPU never limited the game (I had a Geforce 1060 which wasn't so bad but definitely had its limits.)

I didn't play for a while, but then started playing again after I upgraded to a newer system with a Ryzen 5 5600X. Unlike the Zen+, Zen 2 runs all cores at boost, so that 5600X could run at 4.65GHz all cores. Definitely no CPU limit there. After the problem started I started running the 5600X at a fixed 4.4GHz all cores (at that speed the voltage was low enough that a good HSF could keep up.) Some games ran better that way, but no real difference in this one.

I now have a Ryzen 9 9900X. That also means a new motherboard and new memory (because the 5600 was DDR4, so now I'm on DDR5.) I don't dare run this CPU all cores at a fixed speed (encoding or etc would surely destroy the CPU unless I ran it at like 3GHz or something) but I do have PBO lowerivg voltage a bit, so it has absolutely no issues running full speed for anything but the absolute heaviest of tasks. Needless to say, this game is not one of the things that can push it hard enough to drop. This means during the game there is a constant 5.6GHz on CCD0 and 5.4GHz on CCD1 (don't ask me why it works that way.) The only time any cores drop is when load drops too low, but no core has ever gone above something like 38% max. Unlike the 5600X, the precision boost on this 9900X seems to prefer to go to max speed as much as possible, so it doesn't go down much despite being barely used. For a time I was fooled into thinking that the game would be fine because now it seems to take a lot longer than what it took before before the issue crops up -- or maybe it doesn't and it was just coincidence that I didn't see it for a good while. I'm not even sure anymore. But either way it still happens. And still no CPU core is above 38% when this issue happens.