Currently having a strange problem where wp-signups.php blog creation doesn't always complete and times out, so a blog is not always created. I'm running Nginx + PHP-FPM + Varnish on Ubuntu over 2 servers. This is becoming frustrating as the rest of my Multisite installation is running extremely quickly.
My initial thoughts were my Nginx/FastCGI/MySQL buffers were too high or too low, but I've tried hundreds of different values with no change. I also thought it might be a posting problem.
Currently hosting 2200 blogs and 8500 users, but I'm not sure it's a DB issue as I have created a couple of other installations on the same servers and they are having the same issue. If you restart Nginx and PHP-FPM it works for about 10 minutes and then it slows down to a crawl when creating a new blog. I've not tried creating new users through wp-signup.php as I have a custom front end for user creation.
I've enabled PHP/Nginx/MySQL slow logs but it doesn't give me any information on where it is holding up. I also don't want to have to increase my timeouts over 60 seconds due to the size of the installation and other sites I host.
Any words of wisdom on this would be greatly appreciated, so I can stop pulling my hair out.