Custom String vs D String performance

[Patric via Digitalmars-d-learn]

I´m playing remaking D functionalities with nogc structs, and to 
at least match D performance.
But in this particular case i´m unable to get near D performance. 
 Can someone point me out what i´m doing wrong, or if there is 
some magic behind the curtains on D strings?

https://dpaste.dzfl.pl/1c981fdc71ac