I could be wrong but, when twighlight had all of the alicorn magic and she flew with that trail really fast, it could be because alicorn magic is composed of unicorn, earth pony and Pegasus magic. Because she had that boost of Pegasus magic, she had the ability to fly fast. In this case may be rainbowdash was born with more Pegasus magic than a normal Pegasus would be born with, just like Twighlight was born with more unicorn magic than normal, making her magic powerful. May be that's why rainbowdash can fly faster than most pegasi and do a sonic rainboom.