The PC version of Nier: Automata has two very bizarre problems that modders managed to fix within days.
Nier: Automata's PC version is far from being a bad port, but there are a couple of issues PC players have been having since launch that are not very common in these cases.
First of all, the game seems to have a bug that prevents it from rendering at the correct screen resolution. For instance, if you set the resolution in-game to 1080p, Automata will render gameplay at 900p and stretch it to 1080p. Some had better luck running it in windowed mode, but it was never clear what was causing this.
The other issue was typical of some PC ports, but equally perplexing here. Basically, Automata's performance, though capped at 60fps, was erratic on PCs with hardware well beyond the recommended settings. Lowering the settings would have a minimal impact, too, which is never good.
In an attempt to solve the reported problems, modder Kaldaien created a very small installer that takes care of both of them. His mod, which you can grab from the Steam forums, is easy to use but requires you to switch your game to the Testing Branch, which you can do under 'Beta' from the game's 'Properties.'
Kaldaien's creation allows Automata to render at the correct resolution. As to how it improved the graphics performance, this can be attributed to giving users control over the Global Illumination setting - a previously hidden option identified as the cause for the performance hit.
You can adjust the value of Global Illumination and more by pressing Ctrl + Shift + Backspace. Setting it to 16, its lowest value, resulted in up to 60% performance increase for many, and double that on mobile GPUs.
If you're only looking for a quick but more complex fix for only the Global Illumination issue, modder DrDaxxy created some code that brings the setting to the same value. You can get the code from here and use it in a DLL. If this sounds too complicated, stick to the Kaldaien's mod instead.
Quite a few reports on Reddit and NeoGAF indicate the mods are bang on the money, so hopefully they'll have a positive effect for you as well. As always, in these cases it's always worth backing up game files before messing with anything.