Delayed start has two major components:
- Delayed services wait to start until all of the Automatic services have started
- Initially, the threads for delayed services are set to lowest priority.
This greatly reduces the slowdown in responsiveness in user sessions that the services might otherwise cause, because their disk I/O, CPU time, and pace of allocating RAM all ramp up at a more gradual slope than they would otherwise. It helps to avoid the classic "type password then wait 2 minutes" login that we all hate after a fresh boot.
It can also solve some problems if you have, for example, two high-I/O services. You can have one start automatically and the other delayed, or even both delayed, and they may start more smoothly than they would if both configured for Automatic start.
At the moment, it's not really configurable. You can sort of configure chains of service starts by making setting one as dependent on another, even if they technically aren't, i.e. make service 3 dependent on service 2, which is dependent on service 1, then set service 1 to delayed, so they will start in the order 1, 2, 3, once all the Automatic services have started.