What is the bottleneck in a normal use gaming system?
1) What do you call a bottleneck?
If you have a single component that runs at full load while other components are only loaded half way, you have a bottleneck. There are different possible reasons but that is most likely to be it.
2) What can cause bottlenecks?
HDDs and SSDs: Storage is always slow. It has been like that and will be like that for the future.
RAM: Not really an issue in gaming rigs. For notebooks however, the story is different. They slow down for reasons unkown to god himself while using all of their 4GB. Some basic clean up can help here. I work with complex simulations, using 500 of 512GBs RAM is somewhat normal when doing calculations. Here we get to a different limit.
CPU: Depending on what you are doing, stronger cores are better than more cores. Games are normally coded in a cheaper way only taking advantage of one or two cores. Here you can rock with the strong per core team blue. Video work and simulating requires way more completly different calculations. The software used is pricy but the code is better, in most cases it makes use of every core you throw at it. Often the simpler calculations needed to complete the more complex ones done by the CPU can be assisted by fast GPUs.
GPU: Gets simple jobs done, but really, really quick! What ever you want to do, with OpenGL support a fast GPU helps to get the job done. What is interesting about this fancy piece of ad-in card is the different use. Graphic cards optimized for gaming really pack a punch when asked to calculate how the shadows need to be displayed through light rain dropes moved arround by scripted winds. Professional use cards like the Quadro line or the FirePros have massive performance for crunching numbers. They are basically like the calculator you use to help your brain solve math problems. To have numbers to crunch, graphics cards are equipped with their own memory
VRAM: If you have more than 2GB, you will be fine in 1080p. For 1440p, 4Gb (3,5Gb is also okay) will give you some headroom to ad some more filters. 4k is an ever chanching story, one game is fine with only 2 Gigabytes when the next titel can chew more than 6GBs. It comes down to the quailty of the programms used, the drivers and eventually some sort of API.
The User: Yup, you are the biggest bottleneck in the system! Outputting something to display is performance intensive as you want to have more different colors than your eye can differentiate. Input via Keyboard, Mouse, Gamepad or Touchscreen also require calculations. Your brain is estimated to have 1.5PFLOPS of calculation performance but the system used to carry your commands to your hands takes time you do not have.
3) What can you do?
First of all, a hamster in his wheel is a bottleneck for the carrier pigeon. Don´t spend 500$ on a CPU when you want to watch movies in 4k. In my case the CPU was 2/5 the price of the (gaming) graphics card. Ratios between 1/1 and 3/5 (price CPU in 100$ / price GPU in 100$) are normall, busines solutions have similar ratios, except it is 1300$CPU with 3700$GPU.
Another option: As you found this, you will have noticed being on a forum with lots of nice people. Feel free to ask!
I am really sorry for any sort of mistake made in this threat. As this is my personal experience, yours might vary. I am happy to read long texts, so please let me know if there is something to be corrected.