This PC World article might help answer your question: “Meltdown and Spectre FAQ: How the critical CPU flaws affect PCs and Macs.”
It varies widely depending on your hardware, operating system, and workload.
Massive security vulnerabilities in modern CPUs are forcing a redesign of the kernel software at the heart of all major operating systems. Since the issues—dubbed Meltdown and Spectre—exist in the CPU hardware itself, Windows, Linux, Android, macOS, iOS, Chromebooks, and other operating systems all need to protect against it. And worse, plugging the hole can negatively affect your PC’s performance.