Not sure what you are getting at. Naturally more modern processors have a different way to allocate threads, sub-threads and so on differently, while at the same time the efficiency per single core is lower and that's basically what you are seeing - you are using something that necessitates linear/ sequential processing, so threads queue up on a single core while the rest gose unused. Whether that'S something specific with an output format or an effect in your comp we can't know, but unless you can change these things, you will likely have to get used to these behaviors and have to compromise. Unfortunately many things in AE work this way and the only reason you saw better behavior before was probably because your processor just happened to have as many cores only as AE uses threads (for the most part two main threads for rendering operations). A lot of that hasn't even changed in current CC versions despite much better threading in many areas...
While rendering though I can see that all 4 cores of my processor are being used, it shows that they all are just only at 25% CPU usage & my old PC has the exact same amount of cores & also uses all 4 but at 100% usage which causes it to render a good 3 or 4 times faster.
Multiprocessor rendering was removed form CC 2015. Although you can still open your CC 2015 AE project in CC 2014 and use multiprocessor rendering there if you'd like.
Please refer to this blog post: features not available in After Effects CC 2015 | Creative Cloud blog by Adobe
- Render Multiple Frames Simultaneously (Multiprocessing)
The Render Multiple Frames Simultaneously feature has been superseded by the new architecture in After Effects CC 2015 (13.5). The new architecture will allow a future version of After Effects to utilize processor threads and RAM more efficiently than the Render Multiple Frames Simultaneously functionality ever could.
Options related to Render Multiple Frames Simultaneously have been removed from the preferences, and Preferences > Memory & Multiprocessing has been renamed to Preferences > Memory.
If you want to use Render Multiple Frames Simultaneously to speed up final rendering via the render queue, you can still do so by opening the project in After Effects CC 2014 (13.2).
I am using CS6 & on my old PC multi processing is still working & I can still all the CPU's are in use at 100% CPU usage. Whereas using CS6 on my new PC all cores are being used but adobe is seemingly limiting the CPU usage as it's varying from 20% - 99% rather than keeping at a stable CPU usage.
So... what you're saying is that CS6 is somehow discriminating against your new computer?
Is it possible? Ae can open a project newer than itself??