why my tail implementation is much more slower than coreutil ?

[bioinfornatics]

my tail implementation do not provides all features exposed by
coreutil.tail. Just able to read a file no stdout or piping and my D
implementation is slower than coreutil.tail.


---- coreutil tail -----
$ time tail tail.d
                }
                else currentPosition--;
            }
            else
                currentPosition--;
        }
        if( currentPosition != f.length ) writeln( cast(string)
f[currentPosition .. f.length] );
    }
    exit(0);
}

real	0m0.002s
user	0m0.001s
sys	0m0.001s
---- mine tail implementation -----
$ time ./tail tail.d
                else currentPosition--;
            }
            else
                currentPosition--;
        }
        if( currentPosition != f.length ) writeln( cast(string)
f[currentPosition .. f.length] );
    }
    exit(0);
}


real	0m0.011s
user	0m0.004s
sys	0m0.007s

----- Code tail.d ----
import std.mmfile;
import std.getopt;
import std.stdio    : writeln, writefln;
import std.ascii    : isWhite;
import std.c.stdlib : atexit, exit;
import core.runtime : Runtime;

string programName      = "" ;
enum   programVersion   = "0.0.1";

extern (C) void terminateRuntime (){
    Runtime.terminate();
}

bool isNewline(dchar c) @safe pure nothrow {
    return ( c == 0x0A || c == 0x0D )? isWhite( c ) : false;
}


void main( string[] args ){
    void usage(){
        writefln( "%s [OPTION]... [FILE]...", programName );
        writefln( "%s is under GPL v3+", programName );
        writefln( "version: %s", programVersion );
        writeln("    -n, --lines=K   output the last K lines, instead of
the last 10; or use -n +K to output lines starting with the Kth" );
        writeln("    -h, --help      display this help and exit" );
        writeln("    --version       output version information and
exit" );
        exit(0);
    }

    void printVersion(){
        writefln( "%s is under GPL v3+", programName );
        exit(0);
    }

    programName                 = args[0];
    size_t lineNumber           = 10;
    size_t currentLineNumber    = 0;
    ulong  currentPosition      = 0;
    bool   isRunning            = true;

    atexit(&terminateRuntime);

    if ( args.length == 1 ) usage();

    getopt(
        args,
        "lines|n", &lineNumber,
        "help|h" , &usage,
        "version", &printVersion
    );

    foreach( filename; args[1..args.length] ){
        MmFile  f       = new MmFile( filename );
        if ( f.length > 0 )
            currentPosition = f.length - 1 ;
        else
            isRunning = false;

        while( isRunning ){
            if ( currentLineNumber >= lineNumber || currentPosition - 1
< 0)
                isRunning = false;
            else if( isNewline( cast(dchar) f[currentPosition] ) ){
                currentLineNumber++;
                if(currentLineNumber >= lineNumber){
                    isRunning = false;
                    currentPosition++; // do not take tne newline to
output
                }
                else currentPosition--;
            }
            else
                currentPosition--;
        }
        if( currentPosition != f.length ) writeln( cast(string)
f[currentPosition .. f.length] );
    }
    exit(0);
}