Poor Performance or Stuttering in Deus Ex Mankind [SOLVED]
UPDATE2:
I'm also trying to contact Nixxes now to see if I can get a better solution for capping the framerate some other way. Or to implement fullscreen without the added framepacing issues. So check back some time later, or subscribe to the discussion if you're interested.

UPDATE:
No idea how I missed this setting, but thanks to" ÐÜÐĘŞåÜ©Ę™" for pointing out this setting.
Originally posted by ÐÜÐĘŞåÜ©Ę™:
Don't play in exclusive fullscreen. Just play in regular fullscreen. Exclusive has bad stutter for me but regular fullscreen runs smoothly. Turn off msaa completely and play in dx 11.

This confirms my suspicions of a console double-buffer vsync being in place constantly, regardless of the vsync option. Doing this, however, disables all means of V-syncing capabilities. Both through the Nvidia panel and ingame. However, you can run MSI Afterburner and lock the framerate to whatever your refreshrate is in the RivaTuner server here:

https://i.imgur.com/iGLnvf2.png

Proof the game is now running smoothly:
https://i.imgur.com/8HlLa2o.png

Still not perfect, but compared to before it's perfectly playable. Framepacing however without v-sync can still cause some visual anomalies. Will have to try and get V-sync to work without exclusive fullscreen.

Old Post, still relevant for search results and to prove the game itself is to blame
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And don't let anyone tell you it's your computer. After doing some testing it's safe to say this engine has some very apparent framepacing issues regardless of the settings you're using.

I can't recall it being this bad the first time I played through the game, but I immediately noticed it when wanting to revisit the game. Especially in the HUB area's the game just can't keep a consistent, fluid framepacing and it just constantly stutters. I've tried

the ingame Vsync (double/triple)
different frequencies (30/60hz/75hz, seems to get worse the lower you go)
nvidia panel (adaptive/vsync forcing)
setting the ingame settings to the lowest.

System:
4670K @ 4.3ghz
16GB CL11 2400mhz
GTX 1070 (2038/8800)
Installed on SSD
Windows 7 Pro

And the basic problem is illustrated in this picture:
https://i.imgur.com/heMuo4c.png

Don't mind the big spikes in the first quarter, had to tab out a few times. But afterwards you can see the frametimes which are supposed to be around 16.6ms (60hz) deviate from about 16ms to 42ms. The median is obviously 16.6, otherwise it wouldn't be 60fps, but many of the frames lie well above 17ms and even above 20ms. This means the game will display lots of duplicate frames due to the nature of v-sync. If it doesn't get a new frame within 16.6ms the framebuffer won't switch and it will scan the same image.

This results in very poor visual performance and a near constant microstuttering. I can't seem to get rid of it. But the amount of threads I've found where people blindly shoved the blame to people's hardware was rather infuriating. It's obviously the game, since using Nvidia v-sync doesn't solve the issue. And in the picture you can clearly see none of the hardware being maxed out during the high frametimes. Both CPU load (per core) and GPU load are <99%. It maxes out the cores a few times, but there is not clear causal link between CPU usage and high frametime spikes. I'm not running out of memory either.

The game is just very poorly optimized and it runs terribly.

And please don't respond to this thread with "it runs fine" without proof like I've posted above. Show me your frametimes are within a respectable deviation (at 100ms polling).

EDIT:
Here is another run. With better graph y-axis and placing.
https://i.imgur.com/5Y6MJz2.png

Thread HERE: https://psteamcommunity.yuanyoumao.com/app/337000/discussions/0/1692659135915049550/?tscn=1604094239

Don't play in exclusive fullscreen. Just play in regular fullscreen. Exclusive has bad stutter for me but regular fullscreen runs smoothly. Turn off msaa completely and play in dx 11.

So, I would definitely go as far as to say this is required to get a consistently good experience out of this game, and only made this guide so it would be easy to find for anyone later on if the thread gets pushed out of the front page.

All credits go to Majestic for doing the legwork and ÐÜÐĘŞåÜ©Ę™: for coming up with the right configuration.