“Speed of the line” is a concept that plays an important part in landscape design. Often we see strong diagonals at work in Nature- a mountain, a line of trees, or even a roofline of a building. The more steep and uninterrupted that line is, the “faster” it moves the eye- and often it will move the eye right out of the canvas! Here is a little thumbnail showing how that might work.
The eye sails right out the right side of the composition.  The speed of the line can be slowed by creating variety in the line, interruptions so to speak, which slow the movement of the eye and break up the extreme diagonal look of the line. As you can see, the movement along the top line is much different in this example. The slight upward tick in the line creates a little eye “stopper” .
In this thumbnail a stronger stopper in the form of a tree has been added. 
Here a “redirection” stopper has been employed. Although it does not actually break the line it operates to redirect the eye upward into the sky. 
These are all fairly simple examples and solutions. Let’s see how this has been employed by a variety of artists.
Here is Maynard Dixon, iconic painter of western landscapes. That distant mountain on the left side is a stopper and the upward movement of the clouds also works as a redirection.
In this painting the small uptick of the topline on both the right and left side of the hills help slow and redirect the eye.
Hudson River School painter Thomas Cole used those trees on the right of the falls to slow down the descent of the mountains behind.
In a more intimate scene, Hudson River School painter Jervis McEntee used that tree on the left to stop the eye and to redirect into the sky.
So learn to recognize “speed of the line” problems and use some creative variety in the line and “stoppers” to solve the problem!
PS Our online class Composing the Landscape starts October 28, 2022. Learning to compose effectively give us the tools to express our unique vision of the landscape. Nature gives us raw material. It is up to us to make a painting. Join us!
I like being introduced to some new artists and ravishing paintings.