It's actually quite easy theoretically to travel vast distances very quickly. We are only just beginning to understand gravity and how it can be manipulated. Hell, only a few years ago Gravity was supposed to be constant. Now we are beginning to understand gravity waves travel at speed.
Give and object enough mass and it bends space time. Time slows down the closer you get to a mass. For instance time is faster in orbit around the earth than on the surface.
So theoretically you can bend space time (wormhole) to a destination with a large concentration of gravity. You can also slow time. It just requires lots of energy. We haven't figured that bit out yet....Perhaps a breakthrough in dark energy shortly.
Fascinating stuff.