The combined mitigations for Spectre, Meltdown, L1TF, MDS, and Zombieload have decreased the performance of Intel’s CPUs substantially. Phoronix’s Michael Larabel ran tests on the 6800K (Broadwell-E), 8700K (Coffee Lake), and 7980XE (Skylake-SP) with default mitigations applied and found a performance decrease of around 16 percent. With Hyper-Threading disabled, that figure becomes 20 percent, 24.8 percent, and 20.5 percent, respectively.
Larabel also tested two AMD parts, the Ryzen 7 2700X and Threadripper 2990WX. These chips only saw a performance loss of 3 percent.