I wrote a few terrible "finished" novels back in college, which went easily. Then I wrote a lot of fanfic novellas, which varied wildly in how easy they were to write and finish. (One is still sitting around with a chapter that basically says "X happens here, will write it later.") Then I wrote a proper novel over the summer, and it was a wild ride, but went pretty fast. Then I wrote a sequel, which was a hideous slog and took much longer and I discarded at the end as being Just Not Good. The one I'm writing now is going really well, but I'm only two chapters in...
(And I'm not counting any of the unfinished and discarded works, which vary from 2k to 75k on where I stopped and decided they weren't going to work.)
Novels vary. Some spring off the pen like the mythological muses are sitting there doing the writing for you. Some are painful, hideous grinds where every word has to be bled onto the paper. You learn more techniques as you go along, and that both makes things easier (you know how to do it!) and harder (now you know what you did wrong before, and what you should be doing better!). Just the way it goes.
And the funny thing is, how easy the book is to write doesn't correlate to how good the book is. Go figure.