There is an interesting post on Linux Weekly News quoting Greg Kroah-Hartman (who is responsible for the Linux kernel stable releases) suggesting Meltdown and Spectre are comparatively small worries compared to others :
"If you rely on any other kernel tree other than 4.4, 4.9, or 4.14 right now, and you do not have a distribution supporting you, you are out of luck. The lack of patches to resolve the Meltdown problem is so minor compared to the hundreds of other known exploits and bugs that your kernel version currently contains. You need to worry about that more than anything else at this moment, and get your systems up to date first. Also, go yell at the people who forced you to run an obsoleted and insecure kernel version, they are the ones that need to learn that doing so is a totally reckless act."
His full blog post is here which makes for a reassuring read for those keeping their Linux systems up to date.