Perceived rate of increase
Let's assume that you are learning the guitar and today you know one chord. If you learn just one more chord tomorrow, you will have effective double your knowledge! This is one reason that initial knowledge growth is so fast - not because you are learning lots of new information, but because you know so little to begin with. This results in us perceiving our knowledge growth as being much faster at the beginning.
Increasingly focused practice
As we improve at something we naturally zone in on finer and finer skills. This reduces the breadth of what we practice and give the illusion that we are developing slower than at the start.
Unfortunately the last reason has nothing to do with perceptions, it is plain old laziness! Most people will practice less and less as they get better because they lose the initial enthusiasm and perhaps feel that they need less practice to maintain their current abilities.
Learn From the Curve
Try and practice the most important things first. So, if you are learning guitar you might want to learn five of the most commonly used chords first and focus on finger picking later. This will allow you to self correct sooner. As you improve, continually refine what is the most important thing to practice so that you don't rehearse on auto pilot.
Remember that it is better to practice 30 minutes daily than 10 hours once a week. Keep your practices focused and plan what you intend on learning before you begin. All you need to do it move forward each day, it doesn't matter how fast you learn, just keep moving forward!