Getting Bad Gaming Performance Even though CPU and GPU Aren't Being Fully Utilized

Sorry, let me explain further: I use MSI Afterburner to monitor each individual core while I play my game, and the first core gets to about %70 while as I look down the list of cores they all get progressively lower. This is to be expected. I understand that my cpu isn't great with single-threaded performance. Anyways, what you said is what I suspected however my cpu never goes above 40 degrees celsius when gaming even at its peak.

Anyways, when I used FurMark to stress-test the GPU, the fans kicked to 100% after the temp got to about 75 degrees and then the temperature never rose above 81 degrees. (I've read that the 290 should theoretically be able to handle up to 95 degrees) The weird thing is, however, that as soon as I start up the CPU stress-test and run the two simultaneously the GPU automatically downclocks itself to only about 400 MHz and then the framerate during the test goes from about 80fps at 1080p to between 15 and 40, I also read the utilization of the GPU and it was no longer always at 100% anymore, it would go anywhere from 40% to 60%

Yes, I understand "it's stupid to run a CPU and GPU stress test at the same time" some might say but when I did this, I noticed this exact same thing happening in my games as well and in the games, neither the CPU nor the GPU ever reached 100% utilization even though just like during the test it gets downclocked to only about 400-600MHz, nowhere near the 1080MHz clock I have it overclocked to. Keep in mind that when I don't do the CPU stress test on FurMark, the gpu will run and eventually temps will stabilize at 1080MHz, my heat isn't the problem for the CPU and GPU. Building on what I said previously: only a single core reaches 70% and all of the others are nowhere near that with the next highest number usually being about 50% and going down massively each core. (third is only about 20%, fourth only at about 8%, etc)

Curiously I did read somewhere something about the thermal capacitors or something like that on the motherboard not being able to handle the high amounts of heat the CPU can generate but if the CPU is only at about 40 degrees can that cause them to bottleneck it?

Also thanks for the reply.

EDIT: Terribly sorry, I never answered your question before about my GPU. My case has flawless airflow, when building it I made sure one thing I didn't overlook for sure was its ability to stay cool. Also, yes it is a reference model (if by reference model you mean it's not like EVGA or something like that) but it has dual aftermarket fans, however I believe these fans are proprietary of the company I bought my PC from originally. (CyberPowerPC, never going to make that mistake again but at least the fans on the GPU work fine)

Yes, well thanks for confirming my suspicions.