CSES Sum of Two Values TLE

(Problem: edit: CSES - Sum of Two Values)

I coded this problem using an unordered_map, and it time limit exceeded. However, I switched to an ordered map and it was accepted. I read the solution and the solution states that using an unordered_map should work as well. Using an unordered_map also visibly runs faster (on large test cases) when I run it in VSCode. Can anyone tell me why an ordered map was accepted while an unordered map wasn’t?

My code:

#include <bits/stdc++.h>
using namespace std;

int main(){
    ios_base::sync_with_stdio(0); cin.tie(NULL);
    int n; cin >> n;
    int target; cin >> target;
    /*unordered_*/map<int,int> nums;
    for(int i = 0; i < n; ++i){
        int a; cin >> a;
        if(nums.count(target - a)){
            cout << nums[target - a] + 1 << ' ' << i + 1 << endl;
            return 0;
        nums[a] = i;
    cout << "IMPOSSIBLE" << endl;
    return 0;

(note - that link doesn’t actually work)

See here: