After Effects 2023 uses 100% of CPU, not using GPU and struggles to render

If you use a desktop type PC, you change the graphic card to NVIDIA one.

this is expected. only a couple effects use GPU, and multi-frame rendering (MFR) is slower than normal rendering in a number of workflows. GPU usage is essentially nothing, all the time, for every workflow- I don't think it's even used to generate the UI, which is why making sure the renderer ignores a core can speed up performance, by giving headroom so it doesn't just kick your computer in the family silicon every frame.

pre-comping is, for the most part, purely organizational, and is more likely to reduce performance than increase it. note that your new CPU has a lower clock speed, and that your GPU is discrete now. After effects was designed for single core cpu's, and honestly not much else. the further you stray from it, the more you're using patches, updates, and not the core or original code of the program. The people who first coded it are unlikely to still be doing so after 30 years, so in my purely biased and assumption-full concept of it, the people coding it might not fully understand everything going on in the background. It's like taking backstreets instead of the highway. some are faster, but there's more places to hold you up and it's nowhere near as direct.

if the old file or version of AE was prior to 2015 or up to 2017, it likely also had access to multiprocessing, a more advanced, if messier and more incomplete in code, rendering schema. I keep a copy of CS6 kicking around for exactly that reason- I don't need or use the bells and whistles added in the last decade and it can be ported forward if I ever do.