uWSGI worker free, but request' handling has significant delay - uwsgi

Question

I would like to run Django app under uWSGI behind nginx. I've launched 2 uwsgi workers, but I noticed next sad circumstance: when one worker is busy, another worker starts handle request only after 10-15 seconds of waiting.
Configuration is pretty simple.
uWSGI:
uwsgi --socket 127.0.0.1:3031 --wsgi-file wsgi.py --master --processes 2 --threads 1
nginx:
server {
listen 8000;
server_name example.org;
location / {
include uwsgi_params;
uwsgi_pass 127.0.0.1:3031;
}
}
and /etc/nginx/nginx.conf - default value
test Django view:
def test(request):
print('Start!!!')
time.sleep(9999)
print('End')
return HttpResponse()
And wsgi.py has default Django value.
So when I launch all this together and send 2 GET requests then I see in console only one "Start!!!" and only after 10-15 seconds apears second "Start!!!".
I have the same strange behaviour without nginx (with uwsgi --http); with multiple threads for each worker; without "--master" uwsgi option; without django app; with a few uwsgi instances behind Nginx load balancer.
Additional info:
uwsgi version: 2.0.12
nginx version: 1.4.6
host OS: Ubuntu 14.04
Python version: 3.4
Django: 1.9
CPU: 4 cores