Why is elixir GenServer / IPC peformance so different on some of my platforms?

I am seeing huge variations in GenServer performance in some of my platforms and I think it is a result of messaging performance on different architectures. Has anyone seen this before or have any ideas as to what is happening or how I would investigate? I want to make sure that I put a fast system into production rather than a slow one.

Specifically, I wrote a short program that spawns an incrementing GenServer (just keeps a count of how many times it is called), and ten Agents that call that GenServer a million times each. On my i5-9500, the program takes 9.5 seconds, and on my i7-9800X, it takes 48 seconds - about 5x longer. Both systems are running on the same OS (FreeBSD) and the same version of elixir and erlang (built using kerl / kiex).

Some thoughts: The i7-9800X is technically a Skylake-X system, which may have the PAUSE issue, and it also has hyperthreading - maybe those could be part of the issue. Or perhaps there is also a way to change the latency of Erlang / Elixir to respond to messages more quickly. Any ideas / suggestions would be welcome.