I'm pretty confused about why it's beneficial to wait to read the whole compressed file before decompressing. Surely the benefit of beginning decompression before the download is complete outweigh having to copy the memory around a few extra times as the vector is resized?

Streaming prevents many optimizations because the code can’t assume it’s done when run once, so it has to suspend / resume, clone extra data for longer, and handle boundary cases more carefully.

It’s usually only worth it after ~tens of megabytes, but vast majority of npm packages are much smaller than that. So if you can skip it, it’s better.