The WaveVR SDK (used for the Vive Focus Support) uses a dual camera setup. It's the same setup used by the first SDKs of the Vive and the Rift when Unity didn't support VR natively. It's still the case for some VR vendors, for instance Pico and WaveVR.
The problem with this kind of SDK (or the lack of native integration...) is that performances are bad because everything is computed twice. The native integration in Unity compute shadows once and have good optimizations when SinglePass Stereo/Instanced is on.
GunSpinning VR uses a lot of Post effects (aka Post Processing) and I believe that every 3D games have to use them because it's necessary. The Post Processint Stack V2 doesn't work well with this kind of SDK so I've started to make my own very light post processint stack for dual camera SDKs.
Starting with Unity's Fast Bloom, I added an ACES tonemapper and few optimizations, allowing me to use Post processing effects on the Vive Focus with a descent framerate. As you can see on the screenshot, the left eye have the effect and the right eye is clean.