Here’s my conclusion on the recent state of performance regarding Cosmos. Every time that the VR application that opens on the desktop when you launch a game is the active window on the PC, performance tanks inside the VR headset. The SteamVR performance graph becomes purple, and the visual experience within the HMD becomes a blurry, stuttering nightmare- that is, if the application is intensive enough in the first place. I used the VRMark Blue Room to benchmark, and the difference between running the program as the active window vs in the background on the PC was astronomical. I got similar results in Half Life: Alyx and Boneworks.
So, why does it seem like performance has tanked in newer versions of the Cosmos software? I’ve been able to test 1.0.11.2 and 1.0.12.4a side by side (thank you, @C.T.) and I don’t think that it’s the version of the software itself that has affected performance. With both versions, I would get severe lag until I prevented my game from being the active window, which would then return my performance to the silky smooth experience I remember from when I first got my Cosmos Elite in April. Instead, there is probably a more general issue with how any version of the software interacts with newer versions of SteamVR or Windows such that when launching games from within the headset, they now launch as the active window. I am convinced that games did not, for one reason or another, launch this way at one point, since I used to always get the great performance that I now have to take an extra step to achieve.
TL;DR- Running VR applications as the active window on the PC tanks performance in the Cosmos. Launching games from within the headset makes them the active window on your PC by default, which I don’t think used to happen. To fix your VR performance, open a browser tab on your computer to ensure that the VR game isn’t the active window.
All my testing was done on a system with a GTX 1070, R5 3600, and 16 GB of RAM on Windows 10 2004. I cannot guarantee that this fix will work for everyone, but I am positive that it worked for me. This fix would also explain why some people have claimed that changing process priorities in task manager resolved their performance issues. It's not the actual changes they make in task manager that help them, it's that opening task manager involves them stopping the running VR app from being the active window.