What is the deal with stuttering? I tried different video rendering modes, including a new ones from unofficial patch, many other general and specific tips, but still no luck. While monitoring software shows perfectly steady framerate, there still stutters like it drops sometimes to 30-40 fps, very distracting.

Another problem is small UI, I can barely see numbers and crosshair even on 1280x720 while it would be perfect to play up to 4K which is completely impossible. Can it be scaled properly somehow?

Which game specifically? There are several Unreal games on GOG.

Unreal Gold

Using the unofficial OldUnreal patch for Gold v227i, using OpenGL renderer at 1920x1200 (I like 16:10 monitors.... wish it was more standard than 16:9) and it runs super smooth. GTX 1070. i5 2370 (so 8+ year old processor). 8G RAM.
You can try having UsePrecache in Options->Advanced->Rendering->OpenGL Support-> set to True.
This preloads textures I believe, which can help with stutters while moving thru the levels. It adds time when starting a level. Which does not take long on more modern machines. Unless you add the higher res texture packs. Which I used to have before accidentally blowing away my games drive, but currently do not.

Once you have that patch, in the settings there is Font Size you can select double. This primarily changes the menu system to be more readable. There is also the translator size right under that setting which does the translator. I have it at x2 and seems fine.

As to scaling, you would have to use either your video cards scaling or your monitors scaling. I think nvidia cards do a good job here compared to a monitors in most cases.

https://www.oldunreal.com/downloads/unreal/oldunreal-patches/
UnrealGoldPatch227i.7z

I already have unofficial patch, both OpenGL and DX9 have this problem, tweaks didn't helping. Game just runs with perfectly stable framerate but visual stuttering is really bad, like it runs in borderless windowed mode that usually cause stuttering instead of exclusive fullscreen mode, I don't know how to explain, I love older games but I'm not really good at tech stuff... =(

Anyway, I also tried custom DX11 render and it works just fine and looks exactly like the original (with some extensive modern graphics effects disabled). But one problem still persist - cores affinity. From what I know for some reason game runs on 1 core to help with computers, but it makes stuttering on new DX11 even worse, so every time I have to manually open task manager and enable all cores. Can it be fixed somehow?

What to scaling, it works but as I mentioned using 1280x720 as lowest possible 16:9 resolution is a bad choice - HUD remains barely readable and image now blurry and pixelated. Thanks to one good person who did this and one who sent me a link, there is a mod worth to try: www.oldunreal.com/cgi-bin/yabb2/YaBB.pl?num=1581277525

Go into your Nvidia settings for Unreal and change "prerendered virtual-reality frames" from 1 to any other number available and save the change. Then try running the game again.

Thanks! Not virtual reality one, but just pre-rendered frames (aka Low Latency in current driver versions) set to Ultra actually works!

You should not do so. Unreal is an unicore aplication made in the unicore era. If you run it with all cores enabled and OS tries to split it across them, the renderer will very quickly go highwire. That's true for UT99 as well.

Yeah, I already noticed))) Well, so far it was the only way to eliminate even worse DX11 stuttering and I managed to finish couple of levels without any issues. Thanks to Berzerk2002 advise, it works great now with "fail proof" 1-core affinity.

:-)