为什么虚拟机中的磁盘I / O比本机操作系统更快？

Question

#This program reads and writes 1k data to the same files 10000.
import threading
import time

def read_write_file(thread_num):
    wf_name= "w_" + str(thread_num) + ".txt"
    for i in range(0,10000):
        try:
            fd= open("1k.txt","r+")
            w_fd= open(wf_name,"wb")
            str1= fd.read(1024)
            w_fd.write(str1)
            fd.close()
        except:
            print "Error: I/O Exception"
def threads_start(num):
    threads=dict() 
    try:
        for index in range(num):
            threads[index]= threading.Thread(target=read_write_file,args=(index,))
            threads[index].start()
        for i in range(0,4):
            threads[i].join()
    except:
        print "Error: problem in running threads"

start_time= time.time()
threads_start(4)
print time.time() - start_time, " Seconds"

上述代码在虚拟机中比在本机系统中运行得更快。根据我的理解，由于开销，它应该比本机系统慢。为什么它更快？