IIRC there was a bug triggering precompilation each time you re-instantiated manifests, and Pluto notebooks keep a manifest of all packages that they used (in order to have full reproducibility, so it doesn't necessarily match your global environment), so Pluto notebooks would effectively precompile, compile, and run each time. I forget the PR that changed this or I would link it, but IIRC somewhere around v1.9-RC2 this was addressed so the v1.9 release should be much nicer to use for Pluto notebooks. I need to double check this myself though since I was last testing Pluto in some of the betas and reported this behavior.

It is, when the libraries aren't shipped as native libraries, and one depends on the slow LLVM to compile them.

if a small piece of code depends on a few big, complex packages then depending on how things work out, due to the JIT model, you might have needed to essentially recompile all these dependencies at runtime every time. now there are increasingly better precompilation tools to avoid this.

But that needs to happen everytime rustc is updated.