This has nothing to do with performance. Also, there is no such clear distinction in how vectors and lists are used. You still require context to know if a vector is evaluated to itself or not, or if a list is a call. Without context, you can't say looking at [x y] which of x or/and y is being evaluated:

    (let [a [x y]] ...)
    (let [x y] ...)
    (fn [x y] ...) 

As for parenthesis, you have special cases too:

    (defn foo ([] (foo 0)) ([n] ...))

And sometimes, people aren't sure about what to use: https://github.com/bbatsov/clojure-style-guide/issues/64