It is absolutely relevant: If your container is running its own kernel, then it is consuming memory for that kernel, not to mention the CPU overhead of the hosted kernel. Additionally, every container must also boot a complete OS. More overhead. This is why traditional containers use a shared kernel with process group isolation. You pay for what you use. A traditional container only uses what it needs for the app itself, and it starts in a fraction of the time because it doesn't have to boot anything. Performance and cost may be acceptable regardless, but that's not the point. Containers are more efficient.

Based upon responses from Hyper, they appear to address these concerns in a manner I have never heard before. I will certainly be looking into their core technology.