Can this implementation of Damm algorithm be optimized?

[Nestor via Digitalmars-d-learn]

On Thursday, 9 February 2017 at 20:46:06 UTC, Daniel Kozak wrote:
> Maybe you can try use static array instead of dynamic
> static immutable ubyte[10][10] QG10Matrix = ...

I shaved it to this to discard unneccessary time-consuming 
functions:

static immutable ubyte[10][10] QG10Matrix = [
   [0,3,1,7,5,9,8,6,4,2],[7,0,9,2,1,5,4,8,6,3],
   [4,2,0,6,8,7,1,3,5,9],[1,7,5,0,9,8,3,4,2,6],
   [6,1,2,3,0,4,5,9,7,8],[3,6,7,4,2,0,9,5,8,1],
   [5,8,6,9,7,2,0,1,3,4],[8,9,4,5,3,6,2,0,1,7],
   [9,4,3,8,6,1,7,2,0,5],[2,5,8,1,4,3,6,7,9,0],
];

static immutable string number =
   
"0123456789012345678901234567890123456789012345678901234567890123456789";

static int charToInt(char chr) {
   return ((chr >= '0') || (chr <= '9')) ? cast(int)(chr - '0') : 
-1;
}

ubyte checkDigit(string str) {
   ubyte tmpdigit;
   foreach(chr; str) tmpdigit = 
QG10Matrix[tmpdigit][charToInt(chr)];
   return tmpdigit;
}

void main() {
   auto digit = checkDigit(number);
}

I even tried making checkDigit static, but surprisingly this 
increased average execution time by 1ms.

Anyway, the previopus version (modified to benefit from a little 
optimization too) is almost as performant, even though it 
includes multiplication and sum:

static immutable char[] QG10Matrix =
   "03175986427092154863420687135917509834266123045978" ~
   "36742095815869720134894536201794386172052581436790";

static immutable string number =
   
"0123456789012345678901234567890123456789012345678901234567890123456789";

static int charToInt(char chr) {
   return ((chr >= '0') || (chr <= '9')) ? cast(int)(chr - '0') : 
-1;
}

char checkDigit(string str) {
   char tmpdigit = '0';
   foreach(chr; str) tmpdigit =
     QG10Matrix[charToInt(chr) + (charToInt(tmpdigit) * 10)];
   return tmpdigit;
}

void main() {
   auto digit = checkDigit(number);
}

I compiled both with -inline -noboundscheck -release and the 
multidimensional array version does perform 1ms faster for a 
couple hundred runs, but I expected the difference to be much 
more noticeable.

Any idea of what might be happening here?