There is no bottleneck. This is just how AE works. Everything else is irrelevant. AE does not use the GPU for rendering nor are other parts of the programs accelerated. simply look up any thread on "performance optimization" on this forum and elsewhere for more detailed explanations.
The way AE works now is that it looks at a single frame and makes the calculations. The bigger the frame the higher the CPU load. One frame at a time is processed. Temporal effects like frame blending look at more than one frame and make more complicated calculations so the CPU load is higher. Under the current architecture there is no way for these calculations to run your CPU usage up to 100%. AE is working on a re-design of the entire rendering system to make it more efficient but we are not there yet. The best you can do right now is to follow the optimizing your system After Effects suggestions Adobe publishes that come up first in a Google search.
Even before AE's ongoing architecture overhaul began, the Adobe Media Encoder never did make full use of the CPU. The fastest way to render an AE project is to render from After Effects CC 2014 using multiprocessing into an intermediate codec and then use the Adobe Media Encoder to make your deliverable.
That's assuming your project can benefit from multiprocessing as some compositions can't utilize it. (And some computers don't have the specs to handle it, but yours should!) You can open AE projects created with CC 2015 in CC 2014 with no issues (this is what I do).