Associative array of strings perf

[bearophile]

Do you know why an associative array like this:
uint[immutable(char)[]] aa;

Is almost two times faster than a very similar associative array like this?
uint[immutable(E)[]] aa;

Where E is a named typed enum of chars like:
enum E : char { a='a', b='b', c='c', d='d', ... }

Testing code:
http://codepad.org/hzcRH8Bd

Bye,
bearophile