C# SQLite deleting rows really slow

I created a script to delete just about 180 rows in a local sqlite database. Here is my code:

await Task.Run(() =>
{
    sql_con.Open();

    using (var trans = sql_con.BeginTransaction())
    {
        foreach (int i in toDelete)
        {
            cmd = String.Format("DELETE FROM document WHERE docid = '{0}'", i);
            sql_cmd = new SQLiteCommand(cmd, sql_con);
            sql_cmd.ExecuteNonQuery();
        }
        trans.Commit();
    }

    sql_con.Close();
}).ConfigureAwait(false);

I tested it with a stopwatch and Debug.WriteLine...it takes randomly about 2-12 seconds. But it should only take about some milliseconds. The docid column is marked as a index column and in the database are not more than 500 rows or so.

If I use the same script to insert things, it takes just about some milliseconds.

Any ideas?