5 minutes it's a lot, but I have to say I never installed so many add-ons at the same time. FYI, after reading your message, I timed my FSX and it takes exactly 38 seconds to start (the first start of the day), on a default airport with a default airplane. Once I'm there, selecting FSDT JFK takes me another 17 seconds.
The number of installed airplanes surely is a factor, and so is the number of installed sceneries and entries in the scenery.cfg. Also, if your startup flight contains a complex 3rd party airplane, all its gauges will be loaded before FSX starts.
You might gain something by making your startup flight to use a very light default airplane (like the ultralight), go to your airport and THEN loading a complex 3rd party airplane. This because FSX loads all the airplane gauges TWICE: first it load all the airplane+gauges at the first start screen, because your startup flight includes that airplane then, when after you selected your options on the "Create Flight" page, it will unload all of them and, if you kept the same airplane, it will RELOAD the airplane+gauges again together with the rest of the scenery. That's why if your airplane takes a lot to start, you might save some time by having a default airplane in the startup flight, and loading the preferred airplane after you are placed at the airport.