Using std::async slower than non-async method of populating a vector

I am experimenting with std::async to populate a vector. The idea behind it is to use multi-threading to save time. However, running some benchmark tests I find that my non-async method is faster!

#include <algorithm>
#include <vector>
#include <future>

std::vector<int> Generate(int i)
{
    std::vector<int> v;
    for (int j = i; j < i + 10; ++j)
    {
        v.push_back(j);
    }
    return v;
}

Async:

std::vector<std::future<std::vector<int>>> futures;
for (int i = 0; i < 200; i+=10)
{
  futures.push_back(std::async(
    [](int i) { return Generate(i); }, i));
}

std::vector<int> res;
for (auto &&f : futures)
{
  auto vec = f.get();
  res.insert(std::end(res), std::begin(vec), std::end(vec));
}

Non-async:

std::vector<int> res;
for (int i = 0; i < 200; i+=10)
{
   auto vec = Generate(i);
   res.insert(std::end(res), std::begin(vec), std::end(vec));
}

My benchmark test shows that the async method is 71 times slower than non-async. What am I doing wrong?