FidelityFX is the tech developed by AMD, but it's still compatible with Nvidia GPUs. People from Steam reported good result with their games. Since it works well in games, why not DeoVR?
The way FidelityFX works: First your GPU render a lower resolution picture, like 720p, then FidelityFX applies a scaler to make it 1080p, apply some effect to the upscale 1080p so it still looks good.
Therefore, I can increase my rendering scale in SteamVR to higher numbers, like "2888x2468" to "3376x2880" with higher fps, because with FX the rendering frame is actually 2517x1920.
To try it, you just need to download a zip file then play with a Dll file, piece of cake.
1. Download the zip file from here:
https://github.com/fholger/openvr_fsr/releases/tag/fsr_v1.0
Donate to the project if you like their work.
2. Go to this folder:
C:\Program Files (x86)\Steam\steamapps\common\DeoVR Video Player\DeoVR_Data\Plugins\x86_64
Rename the "openvr_api.dll" to something like "openvr_api.origin.dll"
3. Extract the two files from the zip: openvr_api.dll and "openvr_mod.cfg" to this folder. Change the cfg file content to apply your preferences. I change the "renderScale" to 0.67 for my poor G1080.
4. Change the resolution in SteamVR settings. My scale is 0.67, so when I set 3376x2880, the real picture should be 2517x1920 before the FX.
Now run the DeoVR in SteamVR, the new FidelityFX should be working. You might see a better picture with higher fps.