Core i9 Macbook Pro Crippled When Trying To Use It While Exporting

Laptops are not the same as desktops only smaller. In order to get 'similar' performance in a much smaller package with far less heat generated (that they can't dissipate as well) ... a ton of things are "smallified". Everything from motherboard connections and "robustness" to the small chips that are laid out for specific functions on that mobo.

And remember ... you're comparing that laptop to a desktop with double the RAM ... and RAM is a very important part of render/export encoding.

Look at the "discrete" GPU in your laptop ... mine has a GTX2080 in it.  But wait ... that GTX 2080 in my laptop isn't even close to 1/4 the size of a GTX2080 in a desktop ... ? Yea ... and in performance, it can do ... similar ... things ... but ain't gonna match a desktop GTX2080 in most things.

And power use ... make sure you are plugged into the wall when running Premiere, as laptops scale things back when running on battery. Shut off all power management options also.

But understand ... there's choke points built into that laptop to keep things from running too hard, and there's things that the way they're structured simply can't go as fast as we'd like. Which is probably a decent part of the reason that CPU only gets to 20%.

You may well be able to study and find things that can get more speed out of it while rendering/exporting. But rarely will a laptop even come close to a "similar" speced desktop for such work.