AE uses only 15-20% CPU on rendering

After Effects just isn't designed to use all the resources of multi-core processors. It can only go as fast as the slowest task (that could be disk I/O reading the source items, effect calculation or layer compositing) - and many of those tasks are not multi-threaded, so until each one is done the rest of the CPU is just waiting. AE renders frames one by one, so even though you have "spare" resources it won't try to use them. Outputting to H.264 also makes things a bit slower, as it needs to compare several rendered frames in memory to work out the B- and I-frames (as opposed to just writing each frame to disk as an image).

Historically we could tell AE to "render multiple frames simultaneously", but that feature was dropped years ago. There are commercial third-party plugins which try to speed things up by starting several copies of AE, telling each to render part of the timeline as an image sequence, then stitching them back together at the end - but you cannot escape the system bottlenecks, and require a lot more RAM.