[morelisp]

I’m not an Emacs dev so I’ll concede your expertise here - but if it is fundamental to the core navigation and redisplay, why does “stepping down” modes help so much? Fundamental and text mode certainly aren’t great with long lines, but are usually orders of magnitude better (eg a few seconds per command instead of 10s of seconds).

(I have not tried so-long-mode and am mostly on Emacs 26 with some 25.)

[pulisse]

What you're observing is caused by what I'm describing. The features of these other modes are expensive because they do things that implicitly invoke redisplay.

Consider an ostensibly simple operation like determining the column in which a particular character appears. The answer to that isn't the number of characters since the last occurrence of `\n`, because whatever modes are active can inject arbitrary spacing, or display particular strings as something shorter or longer (a trivial example is the mode that causes `lambda` to be displayed as `λ`), or cause some characters to be displayed in a non-roman font where a single glyph is more than one column wide, and so on. To determine the character's column accurately you need to take into account everything that could affect how you'd paint the screen at that position, i.e., run the whole redisplay loop for some portion of a buffer. The richer the set of active modes, the more expensive it is to do that and the more often it is done.