Launching a game means that the entire Steam Window fades and it will become entirely unusable until the game starts. That is not a separate startup window, thats just an embedded element inside the main one. If you accidentally click outside, the game startup gets canceled.
It is a bit more than mildly frustrating when Im waiting on the Vulkan shaders for BFV for like half an hour to process.
I got a setup that maxes out BFV bro. Proton will need to process your shaders every update or so.
If I do have a bottleneck, its my chinese Mobo with a xeon e5-2640 v3. Still, it has DDR4 and M2 SSD, so the bottleneck is usually core speed.