Game performance will always be limited by something.
Usually cpu or gpu.
Such as GTA V will be cpu limited and largely single threaded.
It is unrealistic to expect 100% of both.
And, you really would not want that either.
When you are running a resource at 100% there is no reserve to handle the inevitable surges of demand.
Some games are cpu limited, and some are graphics limited.
How can you tell?
Run YOUR games, but lower your resolution and eye candy.
This makes the graphics card loaf a bit.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
Possibly ram is an issue. If you have insufficient ram, you may be having hard page faults.
When that happens, processing stops until the fault is resolved.
8gb seems to be a bit restrictive these days, particularly if you are doing any multitasking at all.
Another possibility is thermal throttling.
Run HWmonitor or similar.
If your idle temperature is much more than 10-15c. over ambient, your cooler is not doing the job.
If you see a max of 100c. You have throttled until the heat issue is resolved.
That will show low cpu and gpu usage but will be evident as a performance stall.