[barik]

It's possible that both of you are actually right, depending on the package. For example, "plugins" (such as Flymake, or modern variants) can indeed cause the editor to slow to a crawl. Likewise, I had to turn off real-time spelling checking for LaTeX documents in Emacs for this very reason -- on dissertation-size documents it was simply unbearable, and every few sentences the editor would just spin for a bit [2].

[1] http://flymake.sourceforge.net/

[2] http://ergoemacs.org/emacs/emacs_spell_checker_problems.html