Have you investigated memory constraints?
Ryzen is 2 channels; Epyc is 4-8 (depending on CPU). M1 has that stupidly fast/wide setup.
If your Epyc is one of the 4 channel optimized SKUs or is only running in 4 channel mode, you would get pretty close to the quoted ratios on a memory bandwidth test.
Correlation, not causation, but worth looking into.
