Question Why am i getting low frames whilst having low gpu and cpu usage?

[Boylie400]

My specs are:
GTX 2070
Ryzen 7 3800x
16gb ram

I play various games on low settings and i only manage to average around 90 fps with constant drops despite my GPU and CPU usage never going above 30%.
For example i play fortnite on low settings and i manage around 80-110 fps whilst my GPU and CPU usage never exceeds 30%. My drivers are up to date so i'm clueless as to what it is.
Whenever i stream to twitch, my frames drop by 30fps on any game i play and my CPU and GPU usage still never go passed 50%. I feel like my specs are capable of a consistent 144fps so i'm confused on what's the issue, any ideas?

 

[rgd1101]

full system spec? include brand and model of the psu

malware scan the system?
fresh os install for the build?

 

[Boylie400]

full system spec? include brand and model of the psu

malware scan the system?
fresh os install for the build?

the PSU is an "Aero Cool Integrator MX 700W"
I also ran a UserBenchmark so hopefully this helps with the specs, https://www.userbenchmark.com/UserRun/42286225
no malware i'm pretty sure