To a large extent, a plot-centric book should make its central conflict clear as early as possible. If the reader needs to get past hundreds of pages of exposition to find out what the book is "about," that's definitely a structural flaw in the book.
It might not be an insurmountable flaw (if the lead-up is sufficiently well-done); it might also be a flaw that's extremely difficult to correct (e.g. because your speculative element demands a lot of exposition). Nonetheless, it is a flaw, and worth correcting if you can. The extreme form of this is books that "get really good at chapter 12" - in other words, even the author doesn't consider the first 11 books interesting or gripping on their own, so nobody's going to get up to the "good part."
If the book is plot-centric, then the central conflict defines the book. Until you make it clear, the reader doesn't really know why he's reading - he's moving forward primarily on trust, that things will be interesting later. What's worse is, the reader is looking for the plot - he's trying to figure out what's important, where things are heading. Until you tell him, he can get frustrated, or he might start hanging on to various tangents and thinking they're central to the plot. The longer the central plot is absent, the less clear the structure becomes - because you're spending more and more focus on things that won't have the narrative arc, the climax, the payoff, that the plot will.
Take The Girl With The Dragon Tattoo as a negative example. That book starts out with (IIRC) over a hundred pages on journalist Blomkvist's battle with Wennerström, the corrupt industrialist. Only then is Blomkvist called into the Vanger family murder mystery that serves as the center of the book. And once that's solved, we go back to Wennerström, to tie up that plot line. That's awful structure - these two subplots have no connection to each other. It feels like somebody spliced book A into the middle of book B.
This structure makes it tough to tell which we should consider the "main" conflict. Is this a book about a journalist fighting against corruption? Not really, because half the book is spent on an unrelated murder mystery. Well, then, is this the story of an investigation into the past of a twisted, influential family? Kind of, but if that's the case, how come the payoff and climax of the book are all about the completely-unrelated Wennerström? If the book starts and ends with the Wennerström conflict, then that seems to be the defining arc, doesn't it?
So let's talk about how to bring the conflict forward. The trick is to find the core of the conflict, and set that on the table as soon as humanly possible. Everything else - the details, the implications, the consequences - can be layered on above that. Because once the core is clear, the reader will understand why these details are important and how they tie in - vice versa, that isn't the case. Once the core is clear, the reader knows what to focus on, he knows why he's reading, and he can follow along with you more easily, knowing how to tie in new information to the core of your story.
The difficulty is that often, the conflict needs a lot of explanation. So the trick here is to begin from an immediate conflict, something clear and compelling enough to require little upfront exposition, which nonetheless introduces and represents the central conflict. For example:
- Indiana Jones takes a while to reach the Lost Arc, but we see him risking his neck for archealogical treasures right off the bat - and facing off against the same villian he'll be facing later.
- Ender starts out certain he's not going to Battle School, but Ender's Game starts right off with Ender being attacked for being "better" and different, and launches right into both Battle School's untrustworthy interest in him, and into the compelling dynamics between Ender and his siblings. (Notice how little it's necessary to explain about Battle School, about the Buggers, about the whole speculative mileau - all the attention is on the immediate conflicts Ender has to deal with. Of course, these conflicts are carefully chosen to start exposing the setting.)
- Dr. House is absurd, outrageous and cruel right from the beginning of each episode, deliberately creating petty conflicts out of thin air. Part of what this does is start off episodes with immediate conflict, whichs show off the dynamics in the hospital long before the episode has reached any pivotal climax (where such conflict would be natural and necessary). So the tiny conflicts get us ready for the big ones.
So a good way to handle this is as follows:
- Figure out what your central conflict is. It might be a person, it might be a situation, it might be a theme. Know which and what.
- Find a splinter conflict, an offshoot of that main conflict, that can affect your cast right at the beginning. This offshoot should:
- Be something you can introduce right from the start of the book.
- Be extremely easy to understand, at least partially, and to find compelling.
- Be the same "sort" of conflict the central conflict is, or tied tightly to it. When the reader reaches your "main conflict," they should be able to instinctively realize that the "main conflict" and the "initial conflict" are the same thing (or that one is part of the other).
- Good "splinter conflicts" include repercussions of the main conflict, or early "small versions" of the main conflict.
Take The Hunger Games as an example. The central conflict there is the oppression of the Districts by the Capitol. But civil war and cultural revolution are way too much to leap off from right at the start. So the author found a compelling, highly representative conflict - a reality-like survival contest, which is meant (on every level) to exemplify and highlight the Districts' humiliation and powerlessness. The Hunger Games serve as a micro-cosmos, which explains the setting in its entirety, in an exaggerated and intense manner. And even that can be broken down further - in the first book, we know far less about the Games then Katniss does; we only understand the whole procedure as she goes through the whole thing. Because the "big" conflict of the Games is introduced with several immediate conflicts: the difficulty of survival in District 12; the horrific lottery for the Games; the danger to Katniss's family once she leaves. Continue along the book, and each new scene is a new conflict along the same theme. Impress the callous judges. Overcome the despairing drunkenness of the mentor. Survive the initial bloody skirmish. Escape the cruel "professional" contestants. And so on, and so on.
I'll go back to where I started: your book should never be without clear conflict or direction, if at all possible. If you find that it is - consider whether that part could be skipped, condensed, or interwoven more tightly with the plot so that it creates interest and anticipation. You'll never get it all (and you don't want to) - but you'll tighten up your work, and give your reader clear direction and incentive.