People widely report very high memory and CPU usage.

Not all of the memory usage is the fault of Bun/JS. Some of it is likely memory leaks (holding on to data it shouldn't). However, some of the blame does go to using JS. In Rust you can tightly pack your data if you want to, but in JS you can't have nested structs for example. Every object referencing another object is two separate chunks of memory, each with a header, and a pointer from one object to the other. GCs also use more memory. The overhead adds up.

Building a CLI program in JS is a bad choice, and nobody should be defending this decision. Especially since Claude Code is very much able to write Rust code, and they've shown it can port code. Just port Claude Code to Rust directly.