Here is a very thorough discussion at Arstechnica, of not just Meltdown, but Spectre mitigation impact on performance, across Windows, macOS, Linux. I'm posting this comment because recent updates for macOS include (so it is said) patches for Meltdown and Spectre.
The article goes a little more in depth on how impact can be different depending not only OS, but generation of CPU (older CPUs are impacted more than newer ---lake processors. And surprise, how AMD Zen architecture is less impacted than Intel (by happenstance, not because of brilliant forward thinking on AMD's part).
Also, significant difference in discussion of Meltdown vs Spectre patches. The impact of Spectre patches seems to be more significant than Meltdown, particularly for I/O intensive situations. Most of us 'typical' desktop users, won't see much effect from either Spectre or Meltdown patches, but Sys Admins and IT guys running servers definitely have been impacted. And if you run Linux (and a lot of Mac guys also use Linux), the impact of patches is quite significant, according to numerous reports.
What I have yet to read or have explained to me is why macOS , which is a 'nix/kernel based OS like Linux, doesn't APPEAR to be as effected as some of the Linux variants are claiming they are (Red Hat says up to 20% per the article).