These craft fly at incredible speeds within the earth's atmosphere and fly even faster when they get to a certain point out in space. However, they don't fly so fast that flying from this solar sector to another would be achieved without a serious commitment of the crew who fly them. As you can imagine they mostly stay within our solar sector, where they have a clearly defined agenda.
There is a great deal of misunderstanding about how these craft fly as well as how fast they fly, along with their other dimensional characteristics- etc.
How fast these craft fly, can't be defined in terms of light speed because those who fly them would be the first to tell you that the speed of light is greatly misunderstood by contemporary science. There are many variables about space that contemporary science is not yet aware of. They don't use anti-gravity devices, warp space, fly through wormholes or time travel.
The shape of the craft is critical and should give scientists some clues about the structure of space. Aero dynamics, as contemporary science understands it, is not a factor and yes- the U.S. government has been given a craft to study by those advanced being humans we call aliens. I would be surprised if other governments around the world have not been afforded the same opportunity.
These craft do generate heat, but they generate little heat within the dimension that they are flying. When this multi-dimensional craft becomes other dimensional, they are a good distance out in space before they go into "other dimensional" mode. When these craft are in other dimensional mode, they can move to the furthest planet in the solar sector in a few hours. In the earth's atmosphere, they can fly from Corpus Christi, Texas to New York City in about a minute, give or take a few seconds.
These craft become "other dimensional" by pressurizing the craft from the inside which causes the craft to expand just a bit. When these craft are pressurized in this way, they expand right out of our spectrum of perception into another dimension.
Thanks for your question. If you have any other questions or need clarification on alien craft or technology, I would be happy to help!
Best of luck on your journey-.