Re: 25.3 => 26.1, C-mode, slow again

[Winston]

I originally wrote:
>> This problem first appeared several years ago with an earlier Emacs
>> upgrade (23 => 24?).

to which Stefan Monnier <monnier@iro.umontreal.ca> kindly replied:
> The problem you describe is a bit like a traffic jam.
> So even tho you may have seen another problem in an earlier Emacs which
> also looked like a traffic jam, the similarity likely ends there: the
> actual origin of the problem can be extremely diverse.

You may be right.

As part of getting 26.1 to work, I found several (setq default-var)s
that had to be changed to (setq-default var), and it's not uncommon for
function names and variable names to change as major version numbers
increase.  I thought maybe whichever variable I set to fix the problem
last time had had its name changed.

I was actually more surprised that I couldn't identify what I did the
time before.  The main reason I described the variable was in hopes
someone would recognize the description and tell me what *that* variable
was, even if it isn't the cause of my current problem.

As to the current issue, it's not just in C-mode, though it's more
apparent there.  I'm also seeing a very tiny but perceptible lag even in
a small, plain text buffer (such as while composing this reply) when
doing just (forward-line) and (backward-line) that only moves the cursor
and does no scrolling.  That lag wasn't there in 25.3.

Thanks for the reply,
 -WBE