Just came across this.
I guess this is why it's good to read the Debian Mailing List even if you don't use Debian.
One sharp eyed OCaml developer noticed a glitch in short loops of less than 64 instructions affecting Hyperthreading on Kabylake and Skylake CPU's.
Mitigation:
Disable Hyperthreading
Fix:
Install the relevant microcode or BIOS update.
Juicy Details:
https://lists.debian.org/debian-devel/2017/06/msg00308.html