C++ (编辑)我用 Swift 和 C 语言(查找素数)编写了相同的代码，但 C 语言比 Swift 快得多

(There has some Edit in below)
Well, I wrote exactly the same code with Swift and C lang. It's a code to find a Prime number and show that.
I expect that Swift lang's Code is much faster than C lang's program, but It doesn't.

Is there any reason Swift lang is much slower than C lang code?

当我找到第4000个素数时，C郎只用了一秒钟就完成了计算.

这是我写的代码.

有什么解决方案可以加快Swift代码的速度吗？

Swift

import CoreFoundation
/*
var calendar = Calendar.current
calender.locale = .init(identifier: "ja.JP")
*/

var primeCandidate: Int
var prime: [Int] = []

var countMax: Int

print("いくつ目まで？(最小2、最大100000まで)\n→ ", terminator: "")

countMax = Int(readLine()!)!

var flagPrint: Int

print("表示方法を選んでください.(1:全て順番に表示、2:\(countMax)番目の一つだけ表示)\n→ ", terminator: "")
flagPrint = Int(readLine()!)!

prime.append(2)
prime.append(3)

var currentMaxCount: Int = 2
var numberCount: Int

primeCandidate = 4

var flag: Int = 0
var ix: Int

let startedTime = clock()
//let startedTime = time()
//.addingTimeInterval(0.0)

while currentMaxCount < countMax {
    for ix in 2..<primeCandidate {
        if primeCandidate % ix == 0 {
            flag = 1
            break
        }
    }
    
    if flag == 0 {
        prime.append(primeCandidate)
        currentMaxCount += 1
    } else if flag == 1 {
        flag = 0
    }
    
    primeCandidate += 1
}

let endedTime = clock()
//let endedTime = Time()
//.timeIntervalSince(startedTime)

if flagPrint == 1 {
    print("計算された素数の一覧:", terminator: "")
    
    let completedPrimeNumber = prime.map {
        $0
    }
    
    
    print(completedPrimeNumber)
    //print("\(prime.map)")
    
    print("\n\n終わり.")
    
} else if flagPrint == 2 {
    print("\(currentMaxCount)番目の素数は\(prime[currentMaxCount - 1])です.")
}

print("\(countMax)番目の素数まで計算.")
print("計算経過時間: \(round(Double((endedTime - startedTime) / 100000)) / 10)秒")

Clang

#include <stdio.h>
#include <time.h> //経過時間計算のため

int main(void)
{
    int primeCandidate;
    unsigned int prime[100000];
    
    int countMax;
    
    printf("いくつ目まで？(最小2、最大100000まで)\n→ ");
    scanf("%d", &countMax);
    
    int flagPrint;
    
    printf("表示方法を選んでください.(1:全て順番に表示、2:%d番目の一つだけ表示)\n→ ", countMax);
    scanf("%d", &flagPrint);
    
    prime[0] = 2;
    prime[1] = 3;
    
    int currentMaxCount = 2;
    int numberCount;
    
    primeCandidate = 4;
    
    int flag = 0;
    
    int ix;
    
    int startedTime = time(NULL);
    for(;currentMaxCount < countMax;primeCandidate++){
        /*
        for(numberCount = 0;numberCount < currentMaxCount - 1;numberCount++){
            if(primeCandidate % prime[numberCount] == 0){
                flag = 1;
                break;
            }
        }
            */
            
        for(ix = 2;ix < primeCandidate;++ix){
            if(primeCandidate % ix == 0){
                flag = 1;
                break;
            }
        }
            
        if(flag == 0){
            prime[currentMaxCount] = primeCandidate;
            currentMaxCount++;
        } else if(flag == 1){
            flag = 0;
        }
    }
    int endedTime = time(NULL);
    
    if(flagPrint == 1){
        printf("計算された素数の一覧:");
        for(int i = 0;i < currentMaxCount - 1;i++){
            printf("%d, ", prime[i]);
        }
        printf("%d.\n\n終わり", prime[currentMaxCount - 1]);
    } else if(flagPrint == 2){
        printf("%d番目の素数は「%d」です.\n",currentMaxCount ,prime[currentMaxCount - 1]);
    }
    
    printf("%d番目の素数まで計算", countMax);
    printf("計算経過時間: %d秒\n", endedTime - startedTime);
    
    return 0;
}

for ix in 0..<currentMaxCount - 1 {
        if primeCandidate % prime[ix] == 0 {
            flag = 1
            break
        }
    }