The narrator takes a long time because he is contemplating which road to take. That sounds trivial, and it is, IF you only make a surface level reading of the poem.
If you search for a meaning deeper than a guy choosing which road to walk, then you see the poem as an illustration of choices and decisions people make in all aspects of life.
The narrator of the poem is comparing the decision of which road to take to other decisions that he has made in life. The reader is never told about what other specific decisions, but the implication is there.
"Oh, I kept the first for another day!Yet knowing how way leads on to way,I doubted if I should ever come back."