CPU is your bottleneck. It is extremely slow in anything single-core, and up to slightly below-average at everything else. Your GPU is slowing down, most likely, because it is waiting for the CPU to do it's part.
I have this same CPU with a worse GPU and it doesn't have the same issue.
I suggest going into the BIOS and turn off any power savings settings.
Also stress test the CPU and check the frequency alongside with the motherboard temps(monitor temp software tend to confuse the sensors and misreport the CPU core temperatures).
In my crappy board(some gigabyte 78lmt or something like that I don't have that PC right now with me) when having 100% load the frequency drops to 3ghz NOT due to thermal throttling but an issue with the motherboard or the chipset not supporting LLC properly.
To fix this issue. You need to install AMD Overdrive(this is software not hardware) and turn on and off the turbo function of the CPU by going to the Clocks/Voltage tab, the flipside is you need to do it every time you restart(I use it when I want to render something to save rendering times because in the games I play on that PC I don't need to do it, even in BF5).
On the thermal side of things if ANY sensor excluding GPU and HDD is above 70 degrees. You need to replace the stock cooler if you use that one(for the love of God please don't) and make sure there is enough airflow to the VRMs on the motherboard(very very important).
Don't hear them when they say that this CPU is crap. Just there are some very bad motherboards out there and I doubt your Biostar one is a good one. If you want more performance disable turbo boost and set the clock at 3.8ghz. With proper cooling try 4.1ghz and overvolt a bit.
P.S: Use open hardware monitor to monitor everything I said
https://openhardwaremonitor.org/downloads/ I don't trust HWmonitor, as it has been related to malware before.