** UPDATE: I was able to get things rendering much faster by making sure the menus were set to transcode as MPEG-2, 1920x1080, 29.97fps (Precomposed apparently mentions this needs to be set up this way right off the bat in their tutorial), even if the rest of the assets are 720p or standard-def size. My only concern is now if the resulting blu-ray will be sized for the full 1080 versus the 720p size that my project was edited in.
Anyway, another snafu that I found that really semed to fix the speed/rendering issue is to check the durations set for the menus in the Motion tabs -- they should match the durations of the animated background files from Precomposed but I found the first two menu durations were set to over 32 minutes for some reason! So Encore was apparently trying to render out a huge size file to fit that duration! Once I corrected the menu motion durations, the project rendered a LOT faster.