Why does a function has so different performance timing? - c++

Question

I'm learning C++. While solving Leetcode 0009, I wanted to time the function. For 4 different inputs I got the following results consistently:
1
time: 8993
0
time: 88
0
time: 1374
1
time: 1199
My question is why is it taking so long for the first run but not the others? And any other feedback is appreciated.
Here is the code:
//leetcode 0009. Palindrome Number
#include <iostream>
#include <sstream>
#include <string>
#include <chrono>
bool isPalindrome(int x) {
if (x < 0) return false;
std::stringstream ss;
ss << x;
std::string str;
ss >> str;
int len = str.length();
for (int i=0; i<len/2; i++) {
if (str[i] != str[len-i-1]) {
return false;
}
}
return true;
}
void timeit (bool (*f)(int), int x) {
auto s = std::chrono::high_resolution_clock::now();
auto res = f(x);
auto e = std::chrono::high_resolution_clock::now();
auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(e - s);
std::cout << res << std::endl;
std::cout << "time: " << duration.count() << std::endl; //true
}
int main() {
timeit(isPalindrome, 121);
timeit(isPalindrome, -121);
timeit(isPalindrome, 10);
timeit(isPalindrome, 1001);
}