Sorry for my english, it's not mouse lag, but excessive mouse acceleration.
Also my gamepad (Logitech F310) was not detected nor working.
Here's an image of the graphic glitch:

I tested the game using Proton 4.11-13 and it seems to work better.
Mouse acceleration problem is still present, but less than in 5.0-7. Graphic glitch is also still present (so maybe it's a Mesa problem). The gamepad now works flawlessy.
proton 4.11-13x1 2020-05
Compatibility Report
System Information
I confirm:
steam-906100.log.zip
Symptoms
The game runs, but has some problems:
Reproduction
Mouse lagging
The mouse lag is always present and this makes the game difficult to play. Adjusting the mouse sensitivity into the options menu has little impact on mouse lagging and general playability.
Graphics glitching
I found a point in the game were I have a reproducible graphic glitch. I honestly don't know if it's a wanted behavior, but in the initial sequences after you extinguish the fire and check what caused it, when you character touches a panel the screen gets covered with black blocks and also the female protagonist became black, like if some shader isn't working properly.
I have this savegame with the reproducible problem that have to put into this directory:
steamapps/compatdata/906100/pfx/drive_c/users/steamuser/AppData/LocalLow/No Code Studio/Observation/SaveGamesSaveGame.dat.zip
Performance problems
When first launched the game run very well, I played at a smooth 1440p with graphics set to "Good". When I run the game the second time it was very slow, totally unplayable, like if it was running only on one of the cores of my CPU. Mouse was totally unusable, it jumped from a place to another in the half left part of the screen.
I still haven't tried to run the game for the third time.