CSV 读档 + 写档时间慢，求协助寻找问题所在

小弟有一个CSV档案，总共有32000行资料，每一行资料都有8列
我需要从CSV档案读取第6列的每一行的资料并进行处理，且成功运作。

一开始使用1000行测试的时候，执行时间非常快，但是改为执行原档32000行的时候，却变得特别慢，我不是很懂问题所在，希望大家可以给我一些意见。

程式如下：

from datetime import date,datetime
import csv
import codecs
import time
import re
import sys
import os
import jieba
from itertools import repeat
sys.setrecursionlimit(100000000)
input_file = 'Data.csv'
output_file = 'All.csv'
with open(input_file, newline='', encoding='utf-8') as csvfile:
    total_line = len(csvfile.readlines())-1
for_loop = total_line + 1
print(for_loop)
with open(output_file, 'a', newline='', encoding='utf-8') as csvfile:
    csvfile.write('回應作者\n')
for post in range(1,for_loop):
    with open(input_file, newline='', encoding='utf-8') as csvfile:
        reader = csv.reader(csvfile)
        column = [row[5] for row in reader]
        for i, rows in enumerate(column):
            if i == post:
                string = rows
    #print(string)
    string = re.sub('@.*?@', ' ', string, 1)
    flag = 1
    print('Post: ',post)
    while(flag):
        if (string.find('@!@') != -1):
            index = string.find('@!@')
            output_string = string[1:index]
            #print(output_string)
            with open(output_file, 'a', newline='', encoding='utf-8') as csvfile:
                csvfile.write(output_string+'\n')
            string = re.sub(' .*?@', '', string, 1)
            string = re.sub('!.*?@', ' ', string, 1)
        else:
            if(string != ''):
                string = re.sub(' ', '', string, 1)
                output_string = string
                #print(output_string)
                with open(output_file, 'a', newline='', encoding='utf-8') as csvfile:
                    csvfile.write(output_string+'\n')
                string = ''
                if(string == ''):
                    flag = 0
            else:
                print("not found")
                flag = 0