A cached file is typically generated after a user initially visits a page. This creates a delay for the very first user who views it.
The preload allows you to create the cache of all the post, page, category, homepage periodically, which helps much in improving future page loads.
The preload feature does not create cache for the custom types.
It depends on the power of the server. If you have a ordinary hosting package, you should not set more than 4-6. But if you have a VPS, you can set 10-12.
How it works
The preload feature stars to work after delete cache.
When the Preload feature calls the urls, the cache of urls are created automatically. When all the pages are cached, the preload stops working. When the cache is clear, it starts working again.
Manually Speed Up Preload
You can get more detail via the following link