I second Ann. You're doing both upscaling and re-rendering at a different frame rate than what your source footage was to begin with. And that processing eats up your system RAM, and to a lesser extent your CPU. Your Radeon RX 5600 XT, on the other hand, does have hardware H.264 and HEVC encoding albeit only with 4:2:0 chroma subsampling (as it is based on the first-gen RDNA architecture from 2019). What all this means is that you definitely need a new CPU platform in your PC since even if you do upgrade your RAM in your current setup to its maximum supported amount of 64 GB, your current CPU would then become the new bottleneck to your workflow. Plus, the most powerful CPU that you could have upgraded to, the i7-7700K in your case, will have all servicing and support completely ended by Intel itself in a little over one month (not to mention that it would not have been a worthwhile enough upgrade from your i7-6700 to justify your continued use of that now-almost-9-year-old CPU platform). Under the circumstances, save up your money and put it towards a new CPU, motherboard and RAM. And only then it would be worth upgrading to a more powerful GPU (which may also necessitate a new power supply unit as well). By the way, while you're at it, I would also discontinue utilizing your current workflow since no software upscaler performs high quality upscaling. The only quality upscaling solution is an astronomically expensive dedicated hardware upscaler that does not depend on a PC to do its job. Every single software upscaler degrades image quality to some degree. The worst ones degrade your 1080p original when upscaling to "4k" to the point that it looks almost as bad as an average 480p video!
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