Spectre's less of a problem than Meltdown because while it's harder to fix it's also much harder to take advantage of.
As for Meltdown, Apple's fix came out in December and as far as I can tell has had no noticeable effect on anything, at least as far as "everday" amd DAW use is concerned. So with any luck the Windows fix won't either, though to some extent any performance reducing consequences might be more noticeable on older cpus.
The biggest performance problems will be for those running big SQL databases, server farms etc.