iPhone 5 Speed Comparison: Real-Life vs. In-Store; Or, How to Maximize Speed?

So just the other day, I destroyed the screen of my iPhone 5 on a concrete floor, from about a 4 foot drop. Face down. The Horror!

Luckily, I purchased it on launch day and with it I bought AppleCare+, which I had never had before, but as I dislike using a case, it made sense. Thank god I did.

Anyway, while waiting for my genius appointment (which I will note was one of the least pleasant visits I've had, very disappointing) I was naturally playing with the other Apple gadgets in-store. I held in my hand my 16 GB iPhone 5 and a "brand new" display iPhone 5. I doubled clicked the home button on the demo unit and the multitasking bar opened much faster than mine. Probably twice as fast. I repeated test with a couple different units, same results. Closed all apps, same test, same results.

At the time, my iPhone had about 8 GB of offline music, and about 5 GB of other apps, with around 2 GB free, more or less. Is this "bulk" what contributed to the slow down?

I decided to set up my "new" unit as a new iPhone and add apps as I go, trying to keep the amount of weight down to maximize speed. What are your tips for keeping an iPhone running as fast as the in-store units?

tl;dr: iPhone 5 ran much slower than units in store. Now I'm starting new. How do I maximize speed?