Step 1
Click on the Settings cogwheel to the right of the game's Play button in your Steam Library --> Properties --> General --> in the Launch Options put: -dx12 -USEALLAVAILABLECORES

Step 2
*Do this step AFTER already setting all your in-game graphics options*

Go to C:\Users\USER\AppData\Local\HLL\Saved\Config\WindowsNoEditor and open GameUserSettings.ini. with Notepad. Find the FrameRateLimit value (you can search for it by pressing CTRL+F) and change it to 999.000000. This will tell the game to run at a maximum of 999 fps. Then click File --> Save. After this you need to right click the file and go to Properties. Next to "Attributes", checkmark "Read-only" and click Apply.

The reason for doing this is every time the game starts up it tries to set the framerate limit back to 60 fps, so if you don't do this step you are just gonna be locked at a maximum of 60 fps. Also, when you change GameUserSettings.ini to be Read-only the game can't change the framerate back to 60; it can only read the file and then it will use whatever you put in for the framerate limit. If you need to change any of your graphics settings you will need to uncheck Read-only so the game can save the settings to the file. Make sure the framerate limit is correct before re-checking Read-only.

Now that you have set the game to run using DX12, you may or may not notice some stuttering. This can be because of a Windows security measure called Control Flow Guard that actually causes performance issues when using DirectX 12. In this part of the guide we will be telling Windows to not use CFG on Hell Let Loose which will help stop the stuttering. It will not completely fix it though because the truth is Hell Let Loose is just badly optimized for DX12 and the devs really need to fix it. This fix also works for any other games that use DirectX 12 like Fortnite (but will actually completely remove the stuttering in those games as long as the games are well optimized for DX12 in the first place).

Step 1
On your desktop, press the Windows button on your keyboard and then in the search box type in "Exploit protection" and press Enter. Click "Program settings" to switch over to the program settings tab and then click the + next to "Add program to customize".

Step 2
Click "Choose exact file path" and in the file explorer tab that comes up left click the bar to the left of the search bar and copy and paste this in there:
C:\Program Files (x86)\Steam\steamapps\common\Hell Let Loose\HLL\Binaries\Win64
and then press enter. It should now be showing the Hell Let Loose Steam installation folder. Left Click the HLL-Win64-Shipping.exe file and press "Open".

Note: For fixing games other than Hell Let Loose, some may actually have 2 or more .exe files in their installation folder. For this fix to work you always want to choose the one that has DX12 in the name, if such a file exists.

Step 3
In the window that pops up, scroll down to where it says "Control flow guard (CFG)" and check "Override system setting" and then turn it to "Off" and click Apply at the bottom. If you get a Windows Security popup click "Yes".

Alternatively to doing all this you can just completely disable Control Flow Guard for your whole system by going to the top of the "System settings" tab in Exploit protection and turning CFG to "Off by default", but I recommend setting it to just be disabled on a game-by-game basis as outlined above as that is better for your system's security.

If you still notice lots of stuttering:
It might be worth just removing the "-dx12" from the Steam Launch Options since even though the game will be running at a lower framerate it will actually be a smoother experience since you won't get the stuttering. Play with it both ways and see what works better for you.

Watching the intro movies every time the game starts up is annoying, so if you want the game to skip them go to C:\Program Files (x86)\Steam\steamapps\common\Hell Let Loose\HLL\Content and rename the Movies folder to "Movies (rename to just Movies to enable intro movies)". Alternatively you can just delete the folder.