When assessing encoding speed for Audiobooks, there are a lot of factors to consider. With there seeming to be a sudden slow-down, we'll want to explore what might've changed there. Is there anything different about the type of files you have or how they were attained? How big are your original files and is Pass Through enabled (to check this, go into the Build Options accessed from the Finish section)?
In the end, it's usually best for us to see a bit of logging information to find out what all's involved. Would you be willing to start things encoding with a slow project and then generate a System Information report? For that, launch System Information.app from Application/Utilities and then use the Save option. Also, seeing a Sample of the Audiobook Builder processes at that time would be especially helpful to understand what's going on there. For that, with the encoding still going on, launch 'Activity Monitor.app' from Applications/Utilities, select Audiobook Builder from the list of applications that are running, and then hit the 'Sample Process' button at the top of the window. After that, use the Save option and attach the files to an email to
support@splasm.com .