I will endeavor to answer honestly. Part of the answer, not all, but part, lies your complaint: "I could probably compress that book in 20 pages, easily" ... "excruciatingly slow."
It has been many, many years since I touched the first three, but sometimes it is the space between the notes which provides contrast to the notes themselves. Think of music and compare the Phil Spector "Wall of Sound" versus something sparser. "Only in silence the word, only in dark the light ..."
Earthsea is not about action or events or predictable plot beats. It's about the space between such things. Rather than the mastery of magic being D&D's verbal, somatic, and material components, instead lean toward how Vance's Dying Earth series required a wizard to hold an abstract concept in their mind. Here, magic is about the deep understanding created by observation and contemplation, rather than reaction and dissection, and that this is created over long periods.
Imagine a cup. What is ... cupness? Go over the cups you have seen in your mind. Some have handles, some do not, so a handle is not the essence of cupness, but ... an ability to be in hand is. A cup the size of a mountain is not a cup to you, you may not take it in hand. A cup the size of a dust mite is not a cup to you, you may not take it in hand. And so the cup relates to you, and your ability to handle it.
Consider it further. Topologically, with a handle, it might as well be a donut (torus). Topologically, without a handle, it might as well be a ball (sphere). And so mere topology is not the essence of cupness. But shape somehow is. You recognize cupness whether the color or the texture, but by shape. You must explore this further. Cups are fashioned, although one might see them inadvertently created by nature. This goes with their ability to be taken in hand. To fashion something may be art or it may be of utility, so that is insufficient. Consider that your cup has an orientation, such that there is a bottom and there is a top. You have seen inverted cups and from your memory you know when one is upside down. Cupness has an orientation to it. A cup is almost a sleeve, almost a hole, but for that bottom. Which again you discern, is not the place for a hole; if a thing is to be a cup to be taken in hand, that hold belongs on the side to make a handle.
Consider that utility of a cup, fashioned again and again, all over the world. From the thinness of the sides springing up from the "bottom," one is conserving matter, but also maximizing another thing, which is the space that the cup almost contains. It almost contains, but does not contain. It may partially hold. As it almost contains, it means that something may go into and out of the cup. With the orientation you have discovered upon contemplation, you now might see that gravity plays into the almost-containment. A cup is a handle-able thing which almost contains, which holds due to gravity. We now ask what.
The cup is not handcuffs or manacles, to hold a person. The cup is not a cage, to hold an animal or a marble. It has flatness across the top, most often, and one might reason that the thing held is always flat on top, held by gravity. What is held by gravity in an indention, with always a level top? It cannot be a solid object. It cannot be a gas. It could almost be grains of sand, but those form a pile. Ah, then a liquid.
This cup is to be handled, to have a liquid go in and go out, at hand and at rest with its flat bottom, such that it does not tip. It is the size of an adult hand or two child's hands, fashioned. By examination, I reach forth with my hand and may refill it or drink from it.
In Earthsea, wizardry is, in part, to allow yourself enough emptiness to observe a thing in wholeness, to derive all that might be known about it in your own mind, and thereby give yourself a power over it, but not for something so simple as the cup, but as a flower or a shadow or a person. And so in the novels, emptiness provides space for the reader to contemplate and hold what they have read in their mind.