Flask + gunicorn multithreading not working?

Question

Python

function body:

@app.route('/allusers')
def all():
  buff = []

  for u in User.query.all():
    buff.append([u.username, u.email, u.password])
  print("before")
  time.sleep(10)
  print("after")
  return str(buff)

called:

gunicorn --bind 0.0.0.0:6789 main_app:app -w 3 --threads 12

But when you try to load two pages at the same time,
before
after
before
after is printed
and the second page is loaded after 20 seconds, but I would like 10 and
before
before
after
after