Premature optimization

Its been 15 minutes since "Search and replace" started on a 100k file i'm editing in gedit. Its still searching.
Why programmers assume their algorithms will never need to scale?

The offending function is
gtk_source_search_context_replace_all
https://github.com/GNOME/gtksourceview/blob/master/gtksourceview/gtksourcesearchcontext.c