>
> Since it's slow in Fundamental mode as well, it's unrelated to
> font-lock, and thus to this thread.  Display of very long lines is
> known to be slow in Emacs.
>

I believe this is a font-lock problem. If you disable global-font-lock-mode
before you open the file, it opens in seconds. If global-font-lock-mode is
active Emacs stops responding (I gave up after a few minutes).

A quick experiment suggested that the problem is the member
`font-lock-extend-region-wholelines' of `font-lock-extend-region-functions'
that ensures that whenever font-lock highlights keywords in a region, the
full line is rehighlighted (in the case of the example, the full 18M). By
removing it, font-locking the file is done in seconds. (I assume that only
the visible parts of the buffer is highlighted.)

Clearly, simple dropping the function is not a good idea, as keyword
fontification could start anywhere on the line, e.g. in the middle of
identifiers. A better approach would be to extend the region to include
10KB in either direction, plus ensure that the region isn't split in the
middle of identifiers.

   / Anders

Ps. In case anybody is interested in looking further into this, you can use
the tool https://github.com/Lindydancer/highlight-refontification to get
visual feedback on the font-lock refontification process.