I would strongly recommend a faster CPU. It's not something you want to hear, as most people don't like to hear that their hardware is not sufficient, but newer, faster, is better. Sadly, you haven't let us know what your hardware is, so we can not know if your hardware truly is a limiting factor. The other option is to encode heavy grain/noisy sources as x264, as it can handle grain/noise far better than x265 can, currently.
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