It appears alerts are only triggered upon device polling and is not triggered from service polling.
Setup
service_poller_frequency 300 seconds
service_services_frequency: 300 seconds
service_alerting_frequency: 60 seconds
Use Case:
Polling occurs, 30 seconds later a service check is performed and detects SQL Service is in a stopped status. The alerting service will not see this till after the poller complete and runs the alert rule check again after 4 minutes and 30 seconds. Once this is triggered the alerting service may take up to one minute to pickup the alert and send it off. This can cause a major delay in responding to a down system.
I feel moving the Alerting Rule Checks to the alerting service could greatly help in detecting issues quickly.