

Videoplayer smoothly plays high resolution & high bitrate content. No observable jitter, no framedrops, audio & video reliably kept in sync.
App crashes on shutdown and needs to be forcefully terminated. No real issues during runtime, though

Borked
Program failed to launch,
As recommended I manually installed a codec with the following command:
WINEPREFIX=~/steam-library/steamapps/compatdata/451650/pfx ~/.steam/steam/steamapps/common/Proton\ -\ Experimental/files/bin/wine64 ~/Downloads/K-Lite_Codec_Pack_1730_Standard.exe
Afterwards program still failed to launch
Highly Configurable 3D VR Media Player.
Switch to latest beta. I initially had audio but no video on any media. I fixed this by changing the renderer from default to Direct Show ( Video Lan renderer gave me video but no audio for some reason )
As desribed by Töasti, you need to manually install a codec. I installed k-lite standard:
WINEPREFIX=~/steam-library/steamapps/compatdata/451650/pfx ~/.steam/steam/steamapps/common/Proton\ -\ Experimental/files/bin/wine64 ~/Downloads/K-Lite_Codec_Pack_1730_Standard.exe
I also had to switch to the DirectShow renderer in the settings. After this is done, the media player works with no issues.
I further installed vlc 32bit, but that was probably unnecessary.

Drive added via winecfg
On a newer install of Arch, it was unable to access my home folder for some reason despite including the folder in STEAM_COMPAT_MOUNTS. Adding the folder I wanted as a drive in winecfg (with WINEPREFIX set to the pfx folder for Whirligig) fixed the issue.

Lower res videos play fine. Very high resolution videos (especially like what's used for most 3D video) stutter badly. Best to use this app for images and those lower res videos.
In order for the latest Proton versions to access files on another hard drive (if you have multiple drives) you'll need to launch Steam like this:
STEAM_COMPAT_MOUNTS=/path/to/media/files steam
Fix to make DirectShow player backend work (selectable in options). MediaFoundation backend does not work for me.
Download "LAV Installer" from Whirligig website (http://www.whirligig.xyz/player2-1-2). Manually install it in the Steam Whirligig Proton prefix, e.g.:
WINEPREFIX=~/.steam/steam/steamapps/compatdata/451650/pfx ~/.steam/steam/steamapps/common/Proton\ 5.13/dist/bin/wine64 ~/Downloads/LAV\ Installer\ for\ Whirligig\ Installer.exe
mf-install seem to help with performance

Performance can be choppy. Sometimes freezes, especially on videos on higher resolution - I've found some that make it entirely choke. But generally videos play fine so long as they're not too high res.

Performance is probably a bit choppier than on Windows, I don't have a Windows install to test but framerate isn't perfect. Occasionally freezes up for a moment, but only occasionally and so far not while just watching a video, mostly while making certain changes in the settings.