how to concatenate many dataframes in a loop

Question

I have a for loop that generates a new dataframe, then concatenate into a final one, somethink like this:

final_df = pd.DataFrame()
for i in range(2):
    new = pd.DataFrame({'col':[1]})
    final_df = pd.concat([final_df, new])

In my real case, there are millions of "new" dataframe, so this process takes too long. Is there any way to make it more efficient?

Thank you.