That's weird. MangoHud works when I'm running the game in 1920x1200 (but of course I don't want to do that because the stretching seen in >>530371525 annoys me even more than a decreased horizontal FOV would). If I start the game with MangoHud enabled when the game is set to 1920x1080 (or presumably any non-native resolution or aspect ratio), the loading screen before the main menu takes forever (or, at least, longer than my patience; something is clearly wrong in any case). Maybe the Vulkan stuff that draws the MangoHud overlay doesn't like how the game does whatever resolution handling results in the letterboxing I get in 16:9 mode.
As for performance, the game is locked at 30 fps (and apparently unlocking it would cause issues in some parts). I think I'm also seeing some stuttering, and MangoHud actually shows 31 fps half the time, but there's a note on PC Gaming Wiki about SilentPatch fixing frame pacing issues.