The JavaScript extensions, besides requiring a rewrite from the old C model, for a long time were the root cause of performance issues given how much GNOME has outsourced into extensions from what used to be core features in the GNOME 1.x and 2.x days.
I think it's important to note that GNOME extensions aren't a "real" thing as they have no API. They are runtime modifications injected directly into the GNOME environment. This means if their instantiation, cleanup, or operation are not well defined or are buggy, they can break the entire shell. It's not a plugin or add-on system using a dedicated/sandboxed API set to integrate while isolating them from the core platform.
Fair. I do think it's unfortunate how "bare bones" Gnome core has gotten.
It's also a gigantic pain in the ass how major releases often break good extensions. It does seem that is getting better, but for a while there I actively tried to stay on release n-1 because otherwise my extensions would all be broken.
Here is an old example,
https://feaneron.com/2018/04/20/the-infamous-gnome-shell-mem...