flask 入门，渲染页面时快时慢的问题

我用 flask ，就一行渲染页面的代码，渲染的页面也基本纯 html 没有其他的东西，没有外链，为什么有时候要几百毫秒来渲染，多刷新几下又只需要几十毫秒，再刷新又变回几百毫秒反复，对于一个输出验证码图片的接口调用更是一快一慢很有规律……以下是我的代码：

from flask import Flask,Blueprint,redirect,url_for,render_template,send_file,make_response,session
from io import BytesIO
import random
import string
from wheezy.captcha.image import *

home = Blueprint("home",__name__)

@home.route("/")
def index():
    return render_template("home/index.html")


@home.route("/v")
def v():
    captcha_image = captcha([background(),
        text(fonts=['arial.ttf'],
             font_sizes=[46],
            drawings=[warp(),
                rotate(),
                offset()]),
        curve(),
        noise(),
        smooth()],150,45)
    vcode = random.sample(string.ascii_letters + "".join([str(x) for x in range(0,10)]),4)
    image = captcha_image(vcode)
    out = BytesIO()
    image.save(out, 'jpeg', quality=20)
    image.close()
    out.seek(0)
    resp = make_response(send_file(out,"image/jpeg"))
    resp.headers["cache-control"] = "no-cache"
    session["vcode"] = "".join(vcode)
    return resp

if __name__ == '__main__':
    import os
    HOST = os.environ.get('SERVER_HOST', 'localhost')
    try:
        PORT = int(os.environ.get('SERVER_PORT', '5555'))
    except ValueError:
        PORT = 5555
    app.debug = True
    app.run(HOST, PORT)