Hyperthreading in Skylake and Kaby Lake seems to be broken and should be disabled in BIOS/UEFI.
My Laptop is also affected by this bug, but I will try to update the Microcode. Since I'm running Arch Linux I should get the update rather fast.
I underestimated the effect of this bug. For now I disable Hyperthreading completely. Hope a fix will be available, if one is even possible.