Reading quickly is a genuine skill and I do not want to argue against it. Most of what arrives in front of us deserves to be read quickly, or not at all.
The exception
Some writing is constructed so that the second half depends on the first in a way that skimming destroys. You can extract the argument, but the argument was not the point; the sequence was.
There is no way to tell which kind you are holding until you have slowed down enough to find out.