Some event handlers are called immediately when the event occurs. These are called "synchronous' events. An example is DocumentNew. It gets called as soon as the user creates a new document (file buffer).
However, some events are called shortly after the event occurs, usually after a short amount of idle time. These are called "asynchronous' events. They are asynchronous because it would destabilize Source Insight if a user-written macro were to be called at the exact time the event occurred.