[Chuose Osemeka]"I have heard that rendering in CC2015 is slow compared to CC2014,is this true? How can it be avoided?"
After Effects CC 2015.x are the first releases of a major re-architecture effort to replace some of the 23-year-old, under-the-hood architectural assumptions that had put a performance ceiling over previous versions of Ae. The goal is ultimately to make Ae significantly faster on modern hardware, but it's a lot of work and they're not entirely there yet.
On the one hand, the entirely re-written preview engine in Ae CC 2015 is a big improvement over previous releases. On the other hand, Ae CC 2015 is still a bit of a work in progress. Multiprocessing is not currently implemented in Ae CC 2015, and depending on the nature of your project, that can impact render and preview times.
I am using Ae CC 2015 (13.5+) for my daily work in order to take advantage of the new and improved preview system, but I'm still using Ae CC 2014 (13.2) for long renders in order to take advantage of multiprocessing. Ae only changes the project file format on major version numbers, so any 13.x version of Ae can read and write the same AEPs, and it's no problem at all to bounce back and forth between 13.2 and 13.8 all day long.
I think it's multiprocessing that you're referring to when you say you've heard that rendering in Ae CC 2015 is slow compared to 2014. There's a lot that's faster about 2015 -- interactive performance/rendering and expression evaluation especially -- but if you have a multi-core system with a lot of RAM, CC 2014's multiprocessing feature still beats the current release for shorter total render times.
Some things are actually faster in CC 2015.3. The new Lumetri effect uses the GPU to accelerate it and it is definitely faster compared to rendering the same effect in CC 2014 (and the newer effect also has some more features).
I actually haven't opened up CC 2014 at all at my freelance studio since I installed the newest version of AE.
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