How do the aliens get here in such quick time?

The alien ships themselves are not that fast. Sure, they can travel at close to the speed of light but that's it. Most alien craft have a dual propulsion system.

System 1. Is used for routine solar travel and this is what allows them to travel around any area of space at great speed. One trip I took to the moon took mere seconds.

The drive works like a magnetic amplifier. As the sun emits set strands of magnetism, the alien craft take advantage of them riding them like waves. These drives are useless outside a solar system.

System 2. Is used to get from one solar system to another, in other words inter galactic travel. This requires some timing.. not unlike sailing a ship down a river at high tide. When fifth dimensional regions line up - somewhat like the lining up of stars, the ships can use dimension jumping technology. Because the fifth dimension is capable of collapsing into the fourth dimension the distance between say Alpha Centuri and our sun is a matter of metres. Thus, it is a matter of stepping from the curb into the gutter. It's not speed but distance that changes.

The reason that we aren't flooded with intergalactic tourists all the time is that the alignments only take place several times a year. Thus the event known as a "flap" occurs.

This technology is about not how fast you can go, but how short you can make the trip. Imagine that you are the runner in a 1500 metre race. Imagine you had the capability to reduce the space from 1500 metres to one step! That's what dimension crushing technology does.