this is called the “limited by mainthread” ‘thing’.. you can do nothing.
It depends on the application it self how many of them are used and how “actions” are splitted into small-pieces-of-doings which can be parallized.
Of course also not each “action” can be splitted into many “sub-tasks” and then also a 5 million core cpu is not faster.
What you see looks like for most of us and we hope that new implementions like coming DX12 will made it bit better. So.. don’t worry 
And no.. with the affinity you can as exmple ensure that a application x don’t use core y. Or you can shift the load ot a core..
example with an image:
but as @PacificSet90456 already said…
don’t worry about and let windows do that stuff … and the game developers..
PS.: and close the Developer Mode … ( as it say ‘developer’ )