I would think that mIRC goes through the file each time, since if you have two events with the same matches (IE 2x 'on *:text:*:#: {'), only the first one will trigger.
That would be parsing that can happen at any time.
Anyhow, keep in mind that Aliases are listed in the View menu. So, at least with those, mIRC knows what they are and where they are as soon as they are loaded. I wouldn't be surprised if the same is true for all other events because it wouldn't make sense otherwise.
