Tuple vs struct performance?

[bearophile]

Do you know the cause of the large (something like 3 times, latest dmd, -O -release -inline) difference in performance in this program from using a tuple or a struct with the same fields (the two alternative lines near the top)?

http://codepad.org/dLLdgrq8

I have tried in various ways to understand the cause, but I have not found it yet.

Bye and thank you,
bearophile