1. they way get the metadata for a package.
packages are in zip files. zip files have their TOC at the end. So, instead of downloading the entire zip they just get the end of the file, read the TOC, then from that download just the metadata part
I've written that code before for my own projects.
2. They cache the results of packages unzipped and then link into your environment
This means there's no files being copied on the 2nd install. Just links.
Both of those are huge time wins that would be possible in any language.
3. They store their metadata as a memory dump
So, on loading there is nothing to parse.
Admittedly this is hard (impossible?) in many languages. Certainly not possible in Python and JavaScript. You could load binary data but it won't be useful without copying it into native numbers/strings/ints/floats/doubles etc...
I've done this in game engines to reduce load times in C/C++ and to save memory.
It'd be interesting to write some benchmarks for the first 2. The 3rd is a win but I suspect the first 2 are 95% of the speedup.