有個跟速度無關的,
跟程式碼的長度有關,
我覺得你所有的排序可以寫成一個function就好,
帶一個參數進去來決定你要比較的是哪個變數...

struct 在交換的時候，會整塊記憶體複製，應該是慢在這個地方。

temp=*(student+i) ;
*(student+i)=*( student+i+mid);
*( student+i+mid) = temp;

精簡程式的重點在如何共用排序的部分，我使用 sort 函數和 lambda 運算式，
怎麼感覺把 C++ 寫得像 C# 了。

#include <stdio.h>
#include <stdlib.h>
#include <algorithm>
#include <functional>

using namespace std;

struct Grade
{
    char name[10];
    char id[10];
    double ch;
    double en;
};

void print(Grade *arr[], int len)
{
    for (int i = 0; i < len; i++)
    {
        printf("姓名:%d, 國文: %lf, 英文: %lf\n",
               arr[i]->name, arr[i]->ch, arr[i]->en);
    }
}

int main(void)
{
    int len = 6;
    Grade students[len] = {{"張啟東", "A123457", 66, 87},
                           {"朱元璋", "A123458", 89, 90},
                           {"劉與修", "A123456", 80, 98},
                           {"陳明豐", "A123460", 77, 79},
                           {"吳景星", "A123461", 76, 78},
                           {"黃富國", "A123459", 92, 78}};

    //排序使用指標，避免整塊記憶體複製
    Grade *p_students[len];
    for (int i = 0; i < len; i++)
    {
        p_students[i] = &students[i];
    }

    int select = 0;

    while (true)
    {
        printf("1 以國文成績排序\n2 以英文成績排序 \n3 以平均成績排序\n選擇排序項目(打錯則結束!!!)...\n");
        fflush(stdin);
        scanf("%d", &select);

        if (select != 1 && select != 2)
            break;

        //使用 lambda 語法需要 C++ 11 編譯器
        function<bool(const Grade *a, const Grade *b)> compare = NULL;

        switch (select)
        {
        case 1:
            //排序國文
            compare = [](const Grade *a, const Grade *b) {
                return a->ch < b->ch;
            };
            break;
        case 2:
            //排序英文
            compare = [](const Grade *a, const Grade *b) {
                return a->en < b->en;
            };
            break;
        }

        sort(p_students, p_students + len, compare);

        print(p_students, len);
        printf("\n");
    }

    system("pause");
    return 0;
}

把比對部分換成指標函數就可以了，如下

C語言

函數:

bool CompareCh(const struct grade* g1, const struct grade* g2)
{
	return g1->ch < g2->ch;
}

bool CompareEN(const struct grade* g1, const struct grade* g2)
{
	return g1->en < g2->en;
}

呼叫方式:

bool (*compare)(const grade*, const grade*) = CompareCh;
compare(student[0], student[1]);

補充:這網站有介紹排序法的速度、空間和穩定。
除2可以改為位移速度也會快一些 mid /= 2 改 mid >>= 1。

又要變快又要變簡單的..

1. 把你排序時比對兩個資料大小的部份抽出來成為 function pointer
2. 用指標去排序會比較快
3. 10 byte 要裝中文姓名我只能說你很有膽量
4. 不要把 printf 弄成 p, p 是常用的指標變數名
5. stdlib.h 裡面就有個 qsort 了, 自己研究怎麼用
6. typedef 可以讓你少打幾個字

大概就這樣, 下面是隨手改一改的, 預設編譯不過就用 c99 試試:

#include <stdlib.h>
#include <stdio.h>
#include <memory.h>
#include <stdbool.h>

typedef struct grade {
    char *name;
    char *id;
    double ch;
    double en;
    double average;
} grade;

grade student[6] = {
        {"張啟東", "A123457", 66, 87},
        {"朱元璋", "A123458", 89, 90},
        {"劉與修", "A123456", 80, 98},
        {"陳明豐", "A123460", 77, 79},
        {"吳景星", "A123461", 76, 78},
        {"黃富國", "A123459", 92, 78}
};

void average(grade *);
void display(grade *[]);
void sort_grade(grade **, size_t, bool (*)(grade *, grade *));
bool by_id(grade *, grade *);
bool by_chinese(grade *, grade *);
bool by_english(grade *, grade *);

int main(int argc, char *argv[]) {
    average(student);

    short select;
    grade *pStudents[6];
    for (int i = 0; i < 6; ++i) {
        pStudents[i] = &student[i];
    }

    printf("處理前.....\n");
    display(pStudents);
    printf("\n\n");

    printf("處理後.....\n");
    sort_grade(pStudents, 6, by_id);
    display(pStudents);
    printf("\n\n");
    printf("1 以國文成績排序\n2 以英文成績排序 \n3 以平均成績排序\n選擇排序項目(打錯則結束!!!)...");
    fflush(stdin);
    scanf("%hd", &select);

    switch (select) {
        case 1: {
            printf("\n");
            sort_grade(pStudents, 6, by_chinese);
            display(pStudents);
            printf("\n");
            break;
        }
        case 2: {
            printf("\n");
            sort_grade(pStudents, 6, by_english);
            display(pStudents);
            printf("\n");
            break;
        }
        case 3: {

            break;
        }
        default:
            break;
    }

    return 0;
}

void average(grade *student) {
    short index;
    for (index = 0; index < 6; index++) {
        (student + index)->average = ((student + index)->en + (student + index)->ch) / 2;
    }
}

void display(grade *student[]) {
    short i;
    printf("姓名\t\t學號\t\t國文\t\t英文\t\t平均\t\t\n");
    for (i = 0; i < 6; i++) {
        grade *p = student[i];
        printf("%s\t\t%s\t\t%0.2lf\t\t%0.2lf\t\t%0.2lf\n", p->name, p->id, p->ch,
               p->en, p->average);
    }
}

void sort_grade(grade *student[], size_t len, bool (*compare)(grade *, grade *)) {
    grade *temp;
    size_t flog = 0, mid = len, i;
    do {
        mid /= 2;
        do {
            flog = 0;
            for (i = 0; i < len - mid; i++) {
                if (compare(student[i], student[i + mid])) {
                    temp = student[i];
                    student[i] = student[i + mid];
                    student[i + mid] = temp;
                    flog = 1;
                }
            }
        } while (flog == 1);
    } while (mid != 0);
}

bool by_id(grade *a, grade *b) {
    return strcmp(a->id, b->id) > 0;
}

bool by_chinese(grade *a, grade *b) {
    return a->ch > b->ch;
}

bool by_english(grade *a, grade *b) {
    return a->en > b->en;
}

我又來湊熱鬧

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>

#define array_length(x) (sizeof(x) / sizeof(x)[0])

typedef struct {
	char* name;
	char* id;
	float chinese;
	float english;
	float average;
} Student;

int compare_id(const Student* s1, const Student* s2) {
	return strcmp(s1->id, s2->id);
}

int compare_chinese(const Student* s1, const Student* s2) {
	return (int)((s1->chinese - s2->chinese) * 100);  //比較到點數後2個位
}

int compare_english(const Student* s1, const Student* s2) {
	return (int)((s1->english - s2->english) * 100);  //比較到點數後2個位
}

int compare_average(const Student* s1, const Student* s2) {
	return (int)((s1->average - s2->average) * 100);  //比較到點數後2個位
}

void print_results(char* title, Student* students, int length) {
	printf("\n[%s]\n", title);
	printf("姓名     學號     國文    英文    平均\n");
	printf("-------  -------  ------- ------- -------\n");

	for(int i = 0; i<length; i++) {
		Student* std = &(students[i]);
		printf("%-8s %s   %5.2f   %5.2f   %5.2f\n",
			std->name, std->id, std->chinese, std->english, std->average);
	}
}

int main()
{
	Student students[] = {
		{ "張啟東", "A123457", 66, 87 },
		{ "朱元璋", "A123458", 89, 90 },
		{ "劉與修", "A123456", 80, 98 },
		{ "陳明豐", "A123460", 77, 79 },
		{ "吳景星", "A123461", 76, 78 },
		{ "黃富國", "A123459", 92, 78 },
	};

	int n = array_length(students);
	for(int x=0; x<n; x++) {
		Student* std = &(students[x]);
		std->average = (std->chinese + std->english) / 2;
	}


	char* choice_messages[] = {
		"離開程式",	"以學號排序", "以國文成績排序", "以英文成績排序", "以平均成績排序"
	};
	
	int (*compare_fcns[])(const Student*, const Student*) = {
		compare_id, compare_chinese, compare_english, compare_average
	};

	for(;;) {
		printf("\n請選擇排序方法：\n");
		for(int i=0; i<array_length(choice_messages); i++) {
			printf("%1d) %s\n", i, choice_messages[i]);
		}

		int choice = _getche() - '0';
		if(choice == 0) {
			return 0;
		} else if((choice>=1) && (choice<=4)) {
			qsort(students, n, sizeof(students[0]), compare_fcns[choice-1]);
			print_results(choice_messages[choice], students, n);
		} else {
			printf("\n錯誤輸入！（請輸入0至4的數字）\n");
		}
	}
}