Just how fast is a ship going when it is traveling in hyperspace? It turns out, the Lucasfilm Story Group doesn’t worry too much about those details, as Rogue One: A Star Wars Story co-writer Gary Whitta recently revealed that the Star Wars brain trust told him, hyperspace moves exactly as fast as the story requires. According to Whitta…
“The way I heard it was hyperspace moves at the speed of plot, which is, whatever is the most dramatically interesting time it takes to get from somewhere to somewhere else, that’s how long it takes.”
-CinemaBlend interview
However, if you must have in-universe explanations:
Despite being a mature technology and ships having travelled relatively stable hyperspace-routes that were updated constantly, moving through hyperspace was still a dangerous proposition. While generally determined by the distance between two planets, hyperspace travel times between two locations seemingly close to one another could be drastically extended by the need to navigate around stellar hazards, such as asteroid fields and nebulae.
An example of this is the journey from Coruscant to Alderaan. In terms of distance, Alderaan was situated close to Coruscant—the former at approximately 5,000 light years from the Core, the latter at approximately 10,000. However, during the Imperial era, such a journey required roughly sixteen hours of travel due to a section of the route passing through a part of the largely-uncharted Deep Core, where navigation was difficult as a result of the gravity wells produced by the congregation of stars. Ironically, then, it was actually faster to get from Tatooine to Alderaan on the other side of the galaxy. In some cases, intragalactic travels could take days, depending on the distance between two planets and the obstacles between.
-The Wookieepedia article on Hyperspace