Apple has warned its users that the recently revealed Meltdown and Spectre processor vulnerabilities affect all Mac and iOS devices. As such, they should keep everything updated; and only install apps from approved sources (in this case the App Store).
Patches to protect against Meltdown, which is the easier of the two vulnerabilities to exploit, have already been pushed to iOS 11.2, macOS 10.13.2, and tvOS 11.2. Apple developers are still trying to find a way to mitigate Spectre, and a fix should be coming to Safari within the next few days.
To be clear, almost every computing device is affected by both Meltdown and Spectre. Steps are being taken to resolve the problem, and all major computer manufacturers are already releasing patches. For the most part, Meltdown is being dealt with first. This is due to it being the more important of the two. Spectre is believed to be more difficult to exploit.
Some sources are claiming that the fixes for Meltdown and Spectre could result in reduced performance in devices. There has been no observable evidence of this happening as of yet, and Apple has made no mention of the possibility in its update.