I have heard of this a few times. Some people do have really long dreams and stuff. For days or weeks and stuff. I don't really believe you can slow time in a dream, however you can absolutely make dreams seem longer.

The most common thing that makes dreams seem shorter, is that they naturally skip from location to location, and event to event. You can literally wake up and walk out of your bedroom, and arrive at work fully dressed. Saving hours of time. So you might do an entire days worth of activities in five minutes.

You might think that is silly, but it happens in real life too. Some movies and tv shows and even books are able to pack information, and make time seem to last a lot longer. Especially a good book that absorbs you into it.

There is also another trick dreams use, which is false memories. Often times people are made up characters in their dreams, yet some how you see another dream character and you know they are their best friend. And even weirder you remeber you used to hang out at the bar together.

Related to that, its also to have a dream, in which you remeber a dream you had before, yet you had totally forgot that dream. Dreams are easily forgotten, but some times you wake up and realize, "Hey I remeber dreaming that before." So one dream might be connected to an old dream, and you find yourself remebering it.

Lastly, the mind just fills things in from dreams. Some times you don't dream something and all and your mind just fills in the blanks. All the filling in can add a lot more information than was really there, which also can stretch out the dream, making it feel like you did a lot more than you really did.

So all of them can be added and mixed, to make dreams seem like a long time.