Within Static Page Caching is a Queue for caching URIs. Loading a page in ExpressionEngine, especially when the site is receiving heavy traffic, can take an indeterminate amount of time. Further, it does not make sense to cache every page instantly nor to cache multiple pages at the same time, as that can contribute to heavy load on a server. For those reasons, we have the Queue.
With a queue in charge of caching and recaching URIs, we can ensure that Static Page Caching will not hog server resources and that all page caching will happen in an orderly manner.
Pages Not Immediately Cached or Recached
Because of the Queue, the cached URI that you just told the system to renew may not instantly be cached again. The Queue is checked every single time Static Page Caching is touched, which is every single time one of its template tags is processed or its module control panel is loaded. However, it does not process all the queued URIs at once. Instead, it processes the queued URIs in small batches to keep the load on the server load. If you are having many pages being cached on your Site, then the Queue will be full of URIs and you need to be patient as your URI could take a while reach the end of the process list.
Speeding Up the Process
Add URIs tab
Any URIs submitted in the Add URIs section of the module's control panel are cached immediately.
In the module's control panel, the Queue tab allows you to see what URIs are currently in the Queue. If you wish to immediately cache certain URIs, you can do so by selecting them and submitting the form.
There is also the Check Queue tag, which you can put into a highly trafficked template. Every time a page is loaded using that template, the Queue will be checked and processed. This can greatly speed how quickly Static Page Caching processes URIs in the Queue.