Inserting to database using execute_many much slower than using fetch_all

Hi,

I noticed that inserting multiple rows to postgres database with execute_many is much slower than with fetch_all
```
"inserting with execute_many": "Time (hh:mm:ss.ms) 0:00:02.538554",
"inserting with fetch_all": "Time (hh:mm:ss.ms) 0:00:00.249998",
```