Intel Core i5 8400 vs. i5 9400F Meltdown/Spectre/L1TF/MDS Mitigation Impact
With the t-test1 memory allocator benchmarks, the Core i5 9400F was ever so slightly faster than the Core i5 8400 (there's just the +100MHz difference) for both the mitigated and unmitigated results in this synthetic test case.
The network socket performance is one of the areas well known to be affected by these different mitigations and the Sockperf benchmark does a job of exercising those routines. The Core i5 9400F was slightly faster than the i5-8400 but as far as the mitigated vs. unmitigated performance, there was a 44% difference for the i5-8400 while a 35% difference on the i5-9400F.
The socket latency increase from these mitigations was also similar between the CPUs.
Microsoft's Ethr network benchmark shows a similar hit to the performance on both Coffeelake CPUs with their default Linux mitigations,
The Golang HTTP performance was faster on the newer Coffeelake CPU but still a similar hit with the out-of-the-box mitigations.