"Watching paint dry" refers to a period of time when an action is accomplished so slowly as to be considered horrifically boring to the participant. However, this is a highly subjective analogy.
Personally, I love watching paint dry, but only Latex paint, because it dries quickly and always changes color transitioning from wet to dry. And this transformation also contains suspense, because one never knows for sure what the color will actually look like, especially if the sun casts light or shadows on that newly colored wall. Only an artist (painter) and meditator such as myself could possibly endure this exercise.
On the other hand, sitting through nine innings of a baseball game is like "watching paint dry" in the conventional use of this phrase. By the end of the fourth inning, my legs want to power walk me out of the stadium. Of course the father, son, husband or some other male that dragged me there would mightily disagree. The hunched posture, fist pumping, and bellowing are all body language that bespeak of enraptured involvement.
My French husband (not a baseball enthusiast) would have the best resolution to the variations in human perception - a philosophy of life that bypasses heated arguments about how one chooses to spend their time: vive la différence.