Excellent gaming pc not performing well, help!!

[vexzin]

Pc specs;
2x8 gb RAM =16gb 2666mhz
Geforce GTX 1080 FTW DT edition
Intel core i5 8600k 6 cores CPU
CPU liquid cooler NZXT Kraken x62

I play in a BenQ monitor 60hz 1ms 1920x1080.
I think that my pc should be able to AT THE VERY LEAST run most games, a quick example could be fortnite, i should be able to run it without any problem in max settings i assume? The thing is that i can play decently but fps still drop down and i have no max settings.
In BF5 my experience is terrible, it cant even run it decently Lol..
Any ideas of what can it be??

 

[rgd1101]

make and model of the psu?
cpu/gpu temp and usage during the game?
monitor plug into the gpu and not the motherboard?

 

[vexzin]

make and model of the psu?
cpu/gpu temp and usage during the game?
monitor plug into the gpu and not the motherboard?

Power supply = SuperNova 650 G3 650W 80 Plus modular power supply
Monitor= Benq rl2455 24” LCD 16:9 60hz 1ms

I was playing fortnite earlier and my CPU usage was around 80-100% and GPU usage was 40-60%.
Im sure that the monitor is plugged to the GPU. Im using HDMI cable because it has no option for better cables.