open addressing vs chained hashing - c++

Question

Open addressing is usually faster than chained hashing. I am testing my code with successful researches with a low load factor (0.1) but I keep getting best time results for the chained hashing instead of the open addressing. The difference is very very little and sometimes open address is even faster but on an average of 100 inserts the time is better with chained hasing.
I used a vector<string> as hash table for open addressing and a vector<list<string> > for chained hasing and universal hasing for both.
This is the part of the code about successful researches that I am timing:
Chained hashing research:
for (auto it_ric = this->hash[key].begin(); it_ric != this->hash[key].end(); it_ric++)
{
if ((*it_ric) == string)
{
found = true;
}
}
Open addressing:
while (found == false && key != hash.size())
{
if (hash[key] == string)
{
key = hash.size();
found = true;
}
else if (hash[key] == "")
{
key = hash.size();
found = true;
}
else
{
key++;
}
}