Previews are generated asynchronously when they are requested, not when the file is uploaded. If you upload something but never request its thumbnail/rendition it will never be generated.
We don't know whether or not your server can meet the 1 second requirement because everyone's implementation is different and because "200 pages" doesn't say anything about how big the file is. If this is an important metric to the success of your project you need to test it on your hardware in your environment until you are satisfied.
You may have already figured this out, but there is a configurable maximum size of file that he transformation engine will process. Check the docs. And this might be interesting for you: