For Linux 5.0 and as of March this year, the performance impact of enabling Linux kernel mitigations for Spectre/Meltdown against various CPUs with the latest microcode (as of March) are:

Intel: -13% for the Core i9 7980XE, -17% for the 8086K.

AMD: -3% for the 2700X.

Reference: https://www.phoronix.com/scan.php?page=article&item=linux50-...

Phoronix is due to release new benchmarks tomorrow showing full impact from Spectre/Meltdown/L1TF/MDS. There are some initial benchmarks at https://www.phoronix.com/scan.php?page=news_item&px=MDS-Zomb...