how exactly does nginx/php-fpm process requests?

I'm currently testing a new nginx/php-fpm I have setup on a new VPS with 4GB of RAM:

from my php-fpm process pool config:

pm = static
pm.max_children = 10

I have a simple load.php script which has the following simulate a long running mysql query:

<?php
echo sleep(5);
echo "you see me after 5 seconds";
?>

I then throw some load at this script as follows:

ab -n 1000 -c 100 http://mydomain.com/load.php

When I tail the nginx access logs I see a trickle of requests at a time - rather than the high throughput I was expecting.

How exactly does the data flow from nginx to the php backend and back?

If I have 10 static child processes, will the php backend process 10 requests (at 5 seconds each) before then accepting another 10 requests? Certainly from sitting looking at the logs that would appear to be roughly the case - is this correct?

If this is the case and I'm running a php app that if fairly heavy at the DB end (and consequently at the php process time layer), should I simply be increasing the number of child processes to speed up how long it takes to concurrently handle requests?