As far as I remember from previous discussions...
Most of all: Temperature throttling. AVX512 can be a heavy burden.
Furthermore, switching the CPU into and out of AVX modes can be quite time consuming, which has to be considered in the optimization efforts, and it can make it less efficient for lower resolutions.
Please try to read back, and I believe a thread about AVX512 and AMD Ryzen got even separated from this generic x265 encoder thread.
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