When you walk out of the house, how many walls must the signal go through?
Every wall that the signal must pass through will knock down the speed of the signal. That signal loss will vary according to the construction type of the wall, but an "average" wall of sheet rock and 2 by 4s might slow things down by 15-20% or so. So, after 3-4 walls, the signal is going to be pretty slow.
A ceiling is quite a bit thicker than a wall of course.
One of the things that you might not be aware of is that the higher the frequency of the signal, the higher the speed capability. But, as the same time, the higher the frequency of the signal, the weaker the signal. So everything in wireless is a compromise in one way or another.
If you want faster speeds in a remote area, you will likely need to add another AirPort router and connect it back to the "main" router using a wired Ethernet cable connection.