文件I / O代码逐渐减慢

Question

我编写的代码用于解析大量电子邮件（640,000个文件），输出是具有特定日期的电子邮件文件名列表。代码如下：

def createListOfFilesByDate():

    searchDates = ["12 Mar 2012","13 Mar 2012"]
    outfile = "EmailList.txt"    
    sent = "Sent:"

    fileList=glob.glob("./Emails/*.txt")

    foundDate = False

    fout = open(outfile,'w')

    for filename in fileList:

        foundDate = False

        with open(filename) as f:                  
            header = [next(f) for x in xrange(10)]           
            f.close()

            for line in header:            
                if sent in line:
                    for searchDate in searchDates:                                                    
                        if searchDate in line:
                            foundDate = True
                            break

                if foundDate == True:                                                    
                    fout.write(filename + '\n')
                    break

    fout.close()

问题在于代码会很快处理前10,000封电子邮件，但之后开始显着减慢，并且需要很长时间来覆盖剩余的电子邮件。我调查了很多可能的原因，但没找到。我想知道我是否做得不够有效。