An Apple tech note about the Meltdown and Spectre bugs:
Apple released mitigations for Meltdown in iOS 11.2, macOS 10.13.2, and tvOS 11.2. watchOS did not require mitigation. Our testing with public benchmarks has shown that the changes in the December 2017 updates resulted in no measurable reduction in the performance of macOS and iOS … Apple will release an update for Safari on macOS and iOS in the coming days to mitigate [Spectre] exploit techniques. Our current testing indicates that the upcoming Safari mitigations will have no measurable impact on the Speedometer and ARES-6 tests and an impact of less than 2.5% on the JetStream benchmark.
In other words, Apple’s on it. From other sources, it sounds like Macs are less affected by the mitigations for Meltdown because of some specific hardware decisions Apple has made over the past few years.