Running AVX-512 on all cores will lower all core operating frequency, in order to maintain the TDP of the processor.

When you run it in isolation, there's more headroom since only one core can run at the higher freq. and use AVX512 registers.

Also the power license ensures the cores running AVX512 code runs at a lower frequency.

Make sure to guard your AVX512 code block with VZEROUPPER when exiting it