Adobe messed up.
They did not communicate to their customers. The AE team has been trying to tell people about these changes, but not many people even know they have a blog, much less read it: keeping previous versions installed when installing CC 2015 applications
After Effects CC 2015 is not a full version of the software as compared to previous versions in some ways. It includes some very handy new functionality and performance, but it doesn't have everything. This blog post explains some it: features not available in After Effects CC 2015 (13.7)
AE CC 2015 is the first step in a major re-architecture, separating the renderer from the UI for the first time in AE's 20+ year history. This is a big job and it's clearly still in progress. It's a necessary
step for improving After Effects performance, but it's taking a while (and multiple versions) for them to get through it. So, during this time of transition as they're introducing elements of the new
architecture, sometimes it may be easier to use the older version to get a job done. How to Find and Install Previous Version of Adobe Apps in CC 2015
The AE team is too small to get through everything as quickly as it needs to be done. Adobe just doesn't seem to have AE as a very high priority!
A year ago the AE team asked the users (on their blog) for feedback. Did we want them to continue working on AE as usual or would we like them to focus on improving the speed and performance and not give us many new features. We resoundingly told them that we wanted them to focus on speed! So the majority of the (tiny) AE team has been working for over a year to rewrite the core code of AE. The interactivity performance is improved a lot in After Effects CC 2015 (especially if you're updated to 13.7.0), but the rendering performance improvement isn't here yet. As they promised in one of the blog posts I linked, we should have the new renderer in a future version. I hope it's soon!
This is costing me time and money.
What the AE team recommends (and what I do): work in CC 2015 to make use of the snappier interface, better expression handling, etc. and then open that same project file in CC 2014 to render. (No need to "save as" anything weird; AE CC 2014 opens AE CC 2015 projects just fine.) This saves me time because I can work more quickly in CC 2015 - I don't have to wait for the renderer to catch up with me every time I make a change. (This really is a significant improvement over older versions.) And then I just render with CC 2014 so I have the same render speeds as before.