If by 'permanently', you mean even when you intentionally break things, then no. You will always be able to intentionally break things. There is on-going discussion of an auto-prewarm feature. But that doesn't yet exist; and once it does, a super user will always be able to break it.
Presumably you have a use-case in mind other than intentional sabotage of your caches by root. But, what is it? If you reboot the server frequently, maybe you can just throw 'select pg_prewarm...' into an init script?
I didn't express myself well enough. pg_prewarm doesn't help to speed up those queries at all.
Looking at these numbers, I wonder why it takes ~5 secs to answer?