Very likely the former: the majority of proton users, by necessity, play only single player games (I'm one of them).
I'm trying VFIO at the moment, and so far (but it is very early) seems to be working (I also have an integrated video card which greatly simplifies things). I didn't pass through the whole USB controller, just the specific USB peripherals (mouse+kbd and bluetooth controller for audio). I assume this is not done by actual passthrough but there is some layer of indirection; what would be the disadvantage? more latency?
Latency and the ability to switch between the VM and the host. I have a cheap USB input switcher which I've owned for ages for this purpose.
IIUC unless you're running Looking Glass or some other alternative method, passing through each device individually locks them while the VM is running.
I'm trying VFIO at the moment, and so far (but it is very early) seems to be working (I also have an integrated video card which greatly simplifies things). I didn't pass through the whole USB controller, just the specific USB peripherals (mouse+kbd and bluetooth controller for audio). I assume this is not done by actual passthrough but there is some layer of indirection; what would be the disadvantage? more latency?