I'm not familiar with Zstandard and the Zstandard site doesn't discuss how they use threads in compression vs decompression, but I know that for gzip the parall

I'm not familiar with Zstandard and the Zstandard site doesn't discuss how they use threads in compression vs decompression, but I know that for gzip the parallel implementation in pigz is able to split the workload across threads for compression but not decompression. So it could be that similarly in Zstandard, decompression is not an operation that is identically implemented to compression in reverse. There could be differences in thread utilization that shift the balance to being more sensitive to single-core performance.