Optimizing Lisp lexical environments with hashes is going to be a fool's errand. The way you optimize lexical environments is by compiling them, so that there is no searching at all.

The value in the interpreter is that it provides an alternative implementation of the semantics. This becomes particularly valuable in some area that happens to be under-documented, and the compiler and interpreter are found to disagree.

It can be easier to get the semantics right in the interpreter. A simple implementation of environments (and whatnot) reduces the likelihood of bugs and leaves the code readable. Interpreted behavior of special forms can usually serve as the reference implementation for compilation.

If an interpreter is slow, that just means that the build steps for bootstrapping the compiler using the interpreter takes longer.