The metal detectors have NOTHING to do with the long lines. It is the volume of people and all their bags and strollers that have to be examined going through security. I was stopped more than anyone in my family for the metal detectors and it's takes about 15 seconds to go through them, because that includes dropping a bag on a counter, walking through and picking it back up and then continuing on - so don't blame those detectors!!
It depends on which side you are coming from and on what day and when the park opens that affects how long the lines are.
For example on a Tuesday or Thursday when the parks open at 10AM, and they open security at around 8:15, the lines were very long - like backed up to Harbor long on the off-site side of security.
You are going during spring break time - so the park will open at 8AM and that will likely mean security will be flowing smoothly as they will open at 6:30, and most people are still sleeping at that time of day.
If you want to get a good morning experience - then be at the parks at least a half hour before park opening - if you are going to take advantage of any EMH or MM, then be there half an hour before that begins. Then you should be fine. Also, keep in mind, the lines might look long but you'd be surprised how fast they move.
