Phoronix published Intel Xeon vs. AMD EPYC Performance Cost Of Spectre / Meltdown / Foreshadow Mitigations A quote from the article:

One of the most frequent test requests recently has been to look at the overall performance cost of Meltdown/Spectre mitigations on the latest Linux kernel and now with L1TF/Foreshadow work tossed into the mix. With the Linux 4.19 kernel that just kicked off development this month has been continued churn in the Spectre/Meltdown space, just not for x86_64 but also for POWER/s390/ARM where applicable. For getting an overall look at the performance impact of these mitigation techniques I tested three Intel Xeon systems and two AMD EPYC systems as well as a virtual machine on each side for seeing how the default Linux 4.19 kernel performance -- with relevant mitigations applied -- to that of an unmitigated kernel.

 Intel Xeon vs. AMD EPYC Performance Cost Of Spectre / Meltdown / Foreshadow Mitigations