Your performance issue is mainly due to playing on your integrated GPU HD4600 whilst also using it to encode at the same time. It is too much work for it.
Lower the encoder resolution, for example try 720p 60FPS.
1. Updating the HD4600 to the latest driver may help.
Support product highlights, featured content, downloads and more for Intel® HD Graphics 4600
www.intel.co.uk
2. Change your Windows PC power profile to High Performance, Your CPU is currently scaling its frequency. I cannot recall if this will affect the Turbo Frequency on that particular CPU, max it out etc.
***Other suggestions that involve money and risk, and the gains may be minimal or hardware failure***
3. Buy a cheap GFX card. This should help you a lot. Something like a GTX 1030 for example. Market prices are bad though given the current climate.
4. Not sure if you are running a dual memory setup, but if using / playing on the same iGPU as you are, the extra bandwidth will help system performance. 2x identical DIMM sticks are required. Since you have an old system, ebay would be your best bet but there is high risk of buying something that is dud due to age and how it has been handled over the years. Additionally choice is very limited for the type of RAM you need, meaning prices could be very high especially for some of the premium kits that were available at the time.
If you can source a compatible dual DDR kit for free from someone that would be ideal.
5. Do a mild CPU overclock. Your CPU spec is
https://www.intel.com/content/www/u...r-8m-cache-up-to-3-90-ghz/specifications.html Those CPUs can run at the full turbo frequency on all cores 24x7 as long as the CPU cooler is good enough. Setting the windows power profile to high may help you anyway, and
any OC carries a risk of hardware failure. 
