求各位大佬帮忙看看，为什么我这个循环用多线程优化反而速度慢？

def main(trainset,m,n):
for i in range(m,n):
trainset[i] += 1
print('******************%d****************' % m)
def start(trainset):
pool = multiprocessing.Pool(processes=6)
n = 600000
for i in range(6):
pool.apply_async(main,(trainset,i*n,i*n+n))
pool.close()
pool.join()
if __name__ == '__main__':
trainset = [0 for i in range(3600000)]
t1 = time.time()
for i in trainset:
i += 1
t2 = time.time()
start(trainset)
t3 = time.time()
print(t2-t1)
print(t3-t2)
时间如下：
t2-t1 = 0.3051891326904297
t3-t2 = 2.3330767154693604