In Bubble Cup 14, I used std::cin on problem G. Shortest Path and got TLE. See my submission 131334076. Note that I used std::ios_base::sync_with_stdio(false) and std::cin.tie(nullptr) to accelerate it, but it still got TLE. However, I simply changed it to std::scanf and got AC, with an execution time of 342ms. See my submission 131336297.

I tested the efficiency on my computer, but it only took me around 200ms to read $$$10^6$$$ double variables. My computer is Intel i7 8565U. My compiler is gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04). I will show my code and generator below.

#include <algorithm>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
template <typename T>
void checkmax(T &x, T y) {
  if (x < y) x = y;
}
template <typename T>
void checkmin(T &x, T y) {
  if (x > y) x = y;
}
int n;
double a[1000001];
int main(int argc, char const *argv[]) {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(nullptr), std::cout.tie(nullptr);
  std::cin >> n;
  for (int i = 1; i <= n; i++) std::cin >> a[i];
  // std::scanf("%d", &n);
  // for (int i = 1; i <= n; i++) std::scanf("%lf", &a[i]);
  return 0;
}

gen ~~~~~

include <bits/stdc++.h>

std::mt19937 rng( std::chrono::_V2::steady_clock::now().time_since_epoch().count()); template T Rand(T l, T r) { return std::uniform_real_distribution(l, r)(rng); } int n = 1000000; int main(int argc, char const *argv[]) { std::ofstream fout("data.in"); fout.precision(6); fout << n << '\n'; fout << std::fixed; for (int i = 1; i <= n; i++) fout << Rand(-1e6, 1e6) << " \n"[i == n]; return 0; } ~~~~~