> What is/are the real-life use case(s) where such long series of strings causes slowdown?
This is an intermediate state of python buffer when I am trying to build a large-ish dictionary with lists of strings as a value. The most recent time this happened to me is when I was writing a django migration that would organize a list of entities into groups. I did a query to the database, copied the column of string ids, added a double quote to the beginning and to the end and when I tried to group and rearrange the lines together before I wrap them into lists, but emacs became unresponsive.
What I find a bit weird is that it only happens to a long series of bare strings. If I construct an equally long proper python list of strings I don't see this behavior.