Imagine that I create the server "hello.com"
And I create 20 sub-servers:
a.hello.com b.hello.com c.hello.com ...
Suddenly, I have 21 php-fcgi processes along with 4 children each.
That's 21 + (21 * 4) = ONE HUNDRED AND FIVE (105) php processes constantly spinning in memory and stealing cpu and ram.
Really, when we create a new "create sub-server" there must, must, must be an option to say "re-use parent fastcgi daemon" and it should be the default.
That option in turn should make it so that we do NOT create a subdomain fastcgi daemon, but instead tell the nginx sub-server to re-use the one from the parent domain. This is safe and should be the default setup for all new sub-servers, because the parent domain and sub-servers run as the same user so there is ZERO benefit to giving each subdomain their own fastcgi daemon in almost 100% of cases, unless the subdomain gets loads of traffic, or is a different site altogether.
When creating an alias server, it already re-uses the parent server's fastcgi process, so it would be trivial to make sub-servers do the same thing.
The only issue with all of this is that you may not want the sub-server to use the same php.ini as the parent server, but that is why you have an option to make the sub-server spawn its own php-fcgi process with its entirely own php.ini. But don't keep it as the default because it is a HUGE waste of resources in 99% of cases.