I have run an IPv6 tunnel on FreeBSD for about the last ten years, but over the past month or two, I've experienced slowness over IPv6 and have finally tied it to ESXi. FreeBSD 11 with the latest patches running on ESXi 6.5 build 7967591 (Although this was an issue on previous versions too). I have tried multiple ESXi hosts and had the same result with all of them.
I've tried both the Intel NIC and the VMX NIC with the same issues.
If I run from a desktop running the same version of FreeBSD with the same patches, I get the speed I expect.
From ESXi