What would you say to a pastor who wants to rush his preaching of a book of the Bible just to be done with it?
He should be searching his heart as to why he wants to be done. There might be good reasons to shorten the book, so that he doesn't take 8 years to do Romans, but only 1, 2 or 3 years.
I've seen John Stott do masterful exposition of 1 Timothy in 6 sermons, one chapter per sermon. Masterful, better than I could ever do. And I have taken 8 years to preach through Romans. So there are different paces, and there is no "right" or "wrong" about it.
Mark Dever preached one sermon on each book of the Bible and then published them in two books. Amazing! What an amazing achievement and task, and really valuable!
So I want to ask, What is going on in the pastor's heart? Is it because he is losing heart in the Bible? If so, then he has serious work to be done in his spirit with God. Or is it that he is beginning to discern that his gifts are such that long, drawn-out, verse by verse exposition isn't working? The people are not benefiting? He is more gifted in taking larger units and dealing with them more quickly?
But I think the key question here would be: What is going on in his heart? What are the motives? What are the reasons for why he wants to shift? There are good reasons, and if they're good, tell them to the people and go faster.
I once preached fourteen chapters into Acts and quit. I never went back. I feel bad about that. Maybe someday I'll finish Acts, but I got up through chapter fourteen something and then stopped. And that's so long ago I can't even remember why.
So there are good reasons and bad reasons for why you might want to shift gears in the middle of a book. And if they're good, do it.