https://forums.freebsd.org/threads/poor-performance-in-12-1-release-with-vmxnet-3-and-esxi-6-0-0.74417/
Poor Performance in 12.1-RELEASE with VMXNET 3 and ESXi 6.0.0 | The FreeBSD Forums
It took me a while to track this down but I believe there is a problem that is specific to this combination. 12.1-RELEASE (amd64) VMware ESXi, 6.0.0...