I use emacs every day, with a minimal config, and would never describe it as snappy. The entire thing is single threaded so the UI locks up at the slightest provocation, like for example when firing network requests whilst packages are updating.
I remember I looked into a lockup I was experiencing (this was a few years a ago so I forget the mode). Turns out emacs was writing to disk every so often and this was noticeably causing stutters in the main thread.
It really depends on the package, but too many fundamental packages are still operating in the main event loop.
This part seems to be about the User eXperience. (which I agree with, but elisp can start child processes and wait for the results asynchronously, so this is the fault of whoever wrote the code https://www.gnu.org/software/emacs/manual/html_node/elisp/Ou... )