제가 이 코드로 테스트 했을 때는 결과가 다음과 같았습니다.

Debug 빌드:
strlen8  = 3233, time=0.086509
strlen32 = 3233, time=0.041553
strlen   = 3233, time=0.011826

Release 빌드:
strlen8  = 3233, time=0.033875
strlen32 = 3233, time=0.019381
strlen   = 3233, time=0.033616

(단위: 초)
(몇 번 테스트 한 것 중 아무거나 뽑아온 것)

Visual C++ 7.1에서 Win32 빈 프로젝트 만들고 그냥 작성한 것입니다.
제가 테스트 할 때 디스어셈블리 창으로 확인해본 결과, strlen8/32 함수는 인라인 처리 되었었습니다.
이 게시판은 소스 코드 업로드가 안 되므로 부득불 테스트용 전체 코드를 그냥 여기에 적습니다.

[code]
#include <stdio.h>
#include <windows.h>

int strlen8(const char *str)
{
    const char *count = str;
    while (*count) {
        count++;
    }
    return (int)(count - str);
}

int strlen32(const char *str)
{
    const int *count = (int *)str;
    while (1) {
        if ((*count & 0x000000ff) == 0) break;
        if ((*count & 0x0000ff00) == 0) { str--; break; }
        if ((*count & 0x00ff0000) == 0) { str-=2; break; }
        if ((*count & 0xff000000) == 0) { str-=3; break; }
        count++;
    }
    return (int)((char *)count - str);
}

// 시간 측정을 위한 것들
struct PerformanceCounter {
    LARGE_INTEGER StartTime;
    LARGE_INTEGER EndTime;
};

void StartMeasure(PerformanceCounter &time)
{
    time.StartTime.QuadPart = 0;
    time.EndTime.QuadPart = 0;
    QueryPerformanceCounter(&time.StartTime);
}

void EndMeasure(PerformanceCounter &time)
{
    QueryPerformanceCounter(&time.EndTime);
}

double GetElapsedTime(PerformanceCounter &time)
{
    LARGE_INTEGER Freq;
    QueryPerformanceFrequency(&Freq);

    return ((double)time.EndTime.QuadPart - (double)time.StartTime.QuadPart) / (double)Freq.QuadPart;
}



int main()
{
    char text[] =    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    "왜날break!왜날break!왜날break!왜날break!왜날break!왜날break!\n"
                    ;

    PerformanceCounter time;
    int i;
    int len;

    // strlen8 측정 시작
    StartMeasure(time);
    for (i=0; i<10000; i++) {
        len = strlen8(text);
    }
    EndMeasure(time);
    // 측정 끝
    printf("strlen8  = %d, time=%f\n", len, GetElapsedTime(time));

    
    // strlen32 측정 시작
    StartMeasure(time);
    for (i=0; i<10000; i++) {
        len = strlen32(text);
    }
    EndMeasure(time);
    // 측정 끝
    printf("strlen32 = %d, time=%f\n", len, GetElapsedTime(time));

    
    // strlen 측정 시작
    StartMeasure(time);
    for (i=0; i<10000; i++) {
        len = (int)strlen(text);
    }
    EndMeasure(time);
    // 측정 끝
    printf("strlen   = %d, time=%f\n", len, GetElapsedTime(time));

    return 0;
}
[/code]