Efficiency of immutable vs mutable

[Andrew via Digitalmars-d-learn]

This:

On Tuesday, 3 November 2015 at 04:08:09 UTC, TheFlyingFiddle 
wrote:
> __gshared char[4] lookup = ['a', 't', 'g', 'c];

Has the same efficiency gain as immutable, so it looks like a 
thread-local vs global difference and the extra cost is going 
through the thread-local lookup.

Thanks