Pr Resource Use - CPU/GPU

I have a graphics card that is compatible with GPU acceleration and I have GPU acceleration turned on in Settings.

To try and see how Pr uses resources, I've had a system monitor open while rendering a recent project.

The beginning of the project (~35-40 seconds) is all animation.  Assets were created in Ps, imported into Pr with motion added via the Effects panel in Pr.  These same assets are in different places on the timeline at various points in the film.

Looking at the resource monitor, when these animation elements are being rendered, GPU use is high.  This makes sense.  GPU use then drops when standard video segments are being rendered.  This, too, makes sense.  The video card is a Radeon RX580 with 4GB of on card RAM.

When it comes to the CPU, when rendering the animations, CPU use is high (90%+).  I've got an 8-core processor at 3.1 GHz with 16GB of RAM installed (not sure of the RAM clock speed).  When the rendering hits standard video segments, including assets that have been modified in Ae, CPU use drops dramatically into the 30-35% range.  Then, when it comes to the animations again, resource use goes back to 90%+.  Obviously, by using fewer available resources, overall rendering times are slowed.  This project; a 69 minute film with ~25-30% of Ae comps for noise reduction, takes about 16 hours to render in h.264 at ~16Mbps.  I know the Ae comps take longer to render.  Before those were added, the project exported in just over 1 hour.

Is there any insight into why Pr makes such poor use of system resources when rendering standard video segments?  Or is it something that's not universal?

[Moderator note: moved to appropriate forum.]