When Alt+Tabbing, there seems to be quite a high chance of crashes (30-40% from my experience). This chance can be reduced quite a bit by always opening the menu (ESC on keyboard, SELECT button on controller - NOT the controller menu with the Start button, where you slide between World Map, Items etc) before Alt+Tabbing.
Do NOT Alt+Tab while in cutscenes/loading screens, crashes are garantueed.
Also don't stay inside your inventory screen while Alt+Tabbing, the risk of crashes seems even higher there.

Go to "C:\Program Files (x86)\Steam\steamapps\common\The Witcher 3\bin\x64_dx12" (or similar, depends on your install folder)

Right-click witcher3.exe -> Properties -> Compatibility
Disable Full Screen Optimization -> ON
Run this program as an administrator -> ON

Apply

(Might wanna do the same thing to REDprelauncher.exe in the main folder, don't know if that does anything tho)

Go to "C:\Users\[YOURUSERNAME]\Documents\The Witcher 3"
Open dx12user.settings with Notepad
Search for AllowClothSimulationOnGpu (Ctrl+F is the search function)

-> AllowClothSimulationOnGpu=true (you have to overwrite "false" with "true")

Save file and close

For Nvidia cards (don't know about AMD, but there's probably very similar stuff)

Open NVIDIA Control Panel -> Manage 3D Settings -> Program Settings -> Choose "The Witcher 3 (witcher3.exe)" (if its not in the list, search and add it)

Low Latency Mode -> Ultra

Power Management Mode -> Prefer Maximum Performance

Threaded Optimization -> ON

Virtual Reality pre-rendered frames -> 2 !!! probably THE MOST important setting !!!
(might even set it higher than 2, tho I DO NOT KNOW if this is healthy for your PC)
I myself have it on 3 and it reduced CPU temps by ~20-30°C.

(Vertical sync -> Fast !!!This one might not be necessary, try it out if it
helps or even works!!!)

(Texture filtering - Quality -> Performance (might not be necessary, only if severe lag))

Max Frame Rate -> Maybe try without this first, but also maybe consider using this. (I set
it to 90 so my PC doesn't needlessly try to reach my monitors 144Hz frame-rate and set itself
aflame in the process). There's also a recommendation in some forums to set it to half your
monitor's refresh rate or exactly 60 or something, probably worth to keep trying what fits best.

Apply

Open Windows Security -> App and Browser Control -> Exploit Protection Settings -> Programme Settings -> Add Programme to Customize -> Choose witcher3.exe from:

"C:\Program Files (x86)\Steam\steamapps\common\The Witcher 3\bin\x64_dx12" (or your folder)

Edit -> Control flow guard (cfg) -> Override System settings -> turn it OFF

Apply

- Disable/turn OFF MSI Afterburner (or ANY OTHER such software)

- Disable GeForceExperience: Ingame Overlay

(- Disable Steam Ingame Overlay (not a must, but could help))

(- Run Steam as administrator:
C:\Program Files (x86)\Steam (or similar) -> Right-click steam.exe -> Properties ->
Compatibility -> Run this program as an administrator -> Apply)

Only needed if you have a RTX40-series card:

Win 10
Graphics Settings Menu -> Hardware-accelerated GPU scheduling -> ON

Win 11
Display Settings -> Graphics -> Change Default Graphics Settings -> Hardware-
accelerated GPU scheduling -> ON

If modding is too much for you, skip this part for now and see if all the other stuff is doing the trick for you.

These are 2 extremely helpful mods (not a MUST I guess, but can hugely help).
Follow everything the guy has to say (depending on your system/preferences) and give him the LOVE he deserves for saving our very lives.

https://www.nexusmods.com/witcher3/mods/7979?tab=description
https://www.nexusmods.com/witcher3/mods/7426 (NEEDED for the one above)

In Steam Library

Right-click Witcher 3 -> Properties -> Installed Files -> Verify integrity of game files

For Nvidia IF you use DLSS, which is HIGHLY recommended (once again, I'm sure there's also some equivalent for AMD cards).
The reason for this step is that the game does NOT update itself to the latest Nvidia DLSS
drivers and you gotta do it yourself for some stupid reason or another...

Go to https://www.techpowerup.com/download/nvidia-dlss-dll

Download the latest zip-file of nvngx_dlss_[VERSION-NUMBER].zip

Unpack the file (right-click -> Extract Here for example)

Copy nvngx_dlss.dll file to "C:Program Files (x86)SteamsteamappscommonThe
Witcher 3binx64_dx12" and REPLACE the old file



Now go to https://www.techpowerup.com/download/nvidia-dlss-3-frame-generation-dll/

Download the latest zip-file of nvngx_dlssg_[VERSION-NUMBER].zip

Unpack the file (right-click -> Extract Here for example)

Copy nvngx_dlssg.dll file to "C:Program Files (x86)SteamsteamappscommonThe
Witcher 3binx64_dx12" and REPLACE the old file

Restart PC (yeah really, it helps and you just made a ton of changes, soooo ;))

Important in-game options

Display:
Display Mode -> Fullscreen

VSync -> Off

Maximum Frames per second -> Unlimited

Hardware Cursor -> ON

If available:
DLSS Frame Generation -> ON

NVIDIA Reflex Low Latency -> ON (or) ON + Boost (if available)



Graphics (largely depends on preferences and system, so these are just recommendations):

If Ray Tracing ON:
Ray Traced Reflections -> Off
Ray Traced Shadows -> Off (because apparently not necessary)

Anti aliasing -> DLSS (if system supports it)

NVIDIA Hair Works is extremely demanding, try if necessary
!!! Turning it completely OFF might resolve crashing/instability issues !!!

Number of Background Characters also extremely stresses CPU, wouldn't go
higher than medium (even on my water-cooled 13900KS)

The rest to your liking (for example I turned OFF Blur, Motion Blur, Chromatic
Aberration and Vignetting because I don't like these/they just don't look good)

Restart the game after changing the settings, just to be sure. After that check whether everything is the way it should be.

When starting the game and/or while in-game in general, consider closing all browsers and unnecessary background programs, some of them don't just suck off valuable processing power (Chrome for example is a massive black hole for RAM), but could even interrupt/stop processes of the game itself.

There's also the possibility of:

Task Manager -> Details -> Right-click on witcher3.exe -> Set Priority to HIGH

(you will have to do this EVERY single time you start the game, so not the best solution but maybe necessary).

Some guys also suggested downgrading GPU drivers to earlier versions, i.e. I read in one forum that Nvidia 526.98 driver from ~ fall last year made the game stutter way less (didn't do anything for me, but might just help you).

If all these fail, consider a clean install of the game, no mods etc. If you had mods previously, you will have to go to both the Witcher 3 documents folder as well as the steam install one and manually delete everything after uninstalling. (Maybe even clean the registry, in which case please refer to Master Google how to do that for this game.)
!!! BACKUP your saves before that obviously. !!!

Consider adding an exception to the game .exe (and the cdpr-launcher.exe) to your anti-virus program.


Another last resort thing to do is to cut off your internet and ONLY then turn off your antivirus program/firewall whatever (because they could potentially stop certain processes - which I had happen in the past with other games - or slow your game down because of high resource usage).

Once again, ONLY do this after COMPLETELY cutting off your internet connection, otherwise you will NOT be safe, even if you aren't actively using a browser right now.
Also RESTART all of your antivirus/firewall stuff after your gaming session BEFORE reconnecting to the internet.

If nothing works, try DX11.

If you do try DX11, there apparently might be a problem with the CDPR Launcher not using it correctly.
Either go to the game directory and start the game .exe from there, or open the launcher configuration.json and add the line --launcher-skip (I didn't try this personally, so best just Google where and how exactly to do that).

Xamurai
tokito_version2.0
gQx
FTSO

Thank you guys so much for helping all of us!!! Best of luck :D

If anyone knows even more things to do to make this game actually run as it's supposed to, then please let me know in the comments.
Also a like helps others to find this. Thanks!