A general principle of software design is that, within reason, developer time is more expensive than computer time.  Nobody chooses Python over assembly language because it's faster.  They choose it because programmers are wildly more productive with it.

The same is true for declarative languages.  They generally execute more slowly than the same problem solved in C++, sometimes dramatically so.  But for the right kinds of problems, programmers can be wildly more productive with declarative programming than with conventional language.