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If after this memory upgrade your server is not leveraging the available RAM, you may want to leverage one or more caching layers in your stack. It looks like you’re running Nginx and MySQL but it’s unclear what queries are actually hogging the CPU.
If I had to recommend a next step, look into switching your table(s) to the InnoDB engine and set innodb_buffer_pool_size to 4G (make sure that 4GB memory is actually free with a bit more room to spare).
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