Hard to say. I don't have it on my KDE install, but each system and games behave differently with different hardware components.
For starters, you have only 8GB RAM, which is not too much. I have also 8GB and it's OK for the most time, but I configured dynamic swap file and my SSD memory is relatively fast, so it can expand if needed, plus my RAM is also fast.
Secondly, your CPU is i5 but relatively new generation, so this looks fine.
Your GPU can do some gaming but it's not awesome for gaming. So it simply may come down to the fact that both RAM and GPU are struggling with some quick changes when it comes to switching windows. As to the freezes during volume change, this sound like a bug of some sort.
Since you have hybrid graphics, how did you configure it?
Show me output of:
Are you sure your Nvidia is working when you are playing the game? Maybe Intel is doing the work as CPU and GPU and it's been struggling, hence the issues?
If you open Steam and dgo to Help->System Information, what does it say under Graphical Card, Driver?