Re: Performance degradation from long lines

[Alexander Shukaev <emacs@Alexander.Shukaev.name>]

Modes like `so-long' are not useful at all to resolve these issues 
(because, example it does not prevent the initial activation of the 
offending major mode, but rather only disables it afterwards).  The only 
way I found to reliably mitigate the problem in an semi-automated manner 
consists of two parts.  First,

(require 'longlines)
;;
(defcustom global-longlines-threshold
   line-number-display-limit-width
   "\
Maximum line length permitted before applying `longlines-mode'.

See `global-longlines-p'."
   :group 'longlines
   :type 'integer)
;;
(defun global-longlines-p
     (&optional buffer)
   "\
Test all lines of buffer BUFFER for `global-longlines-threshold'.

Return t if `global-longlines-threshold' is exceeded."
   (with-current-buffer (or buffer (current-buffer))
     (when (integerp global-longlines-threshold)
       (save-restriction
         (widen)
         (save-excursion
           (goto-char (point-min))
           (let ((inhibit-field-text-motion t))
             (catch 'break
               (while (not (eobp))
                 (when (> (- (line-end-position 1) (point))
                          global-longlines-threshold)
                   (throw 'break t))
                 (forward-line 1)))))))))
;;
;;;###autoload
(defun turn-on-longlines-mode-maybe ()
   (interactive)
   (when (global-longlines-p)
     (longlines-mode)))
;;
;;;###autoload
(define-globalized-minor-mode global-longlines-mode longlines-mode
   turn-on-longlines-mode-maybe
   :group 'longlines)
;;
(provide 'global-longlines)

and then configure

(add-hook 'after-init-hook #'global-longlines-mode)

Secondly,

(defgroup inhibit-set-auto-mode
   nil
   "Inhibit `set-auto-mode'."
   :group 'files
   :prefix 'inhibit-set-auto-mode-)
;;
(defcustom inhibit-set-auto-mode-functions
   nil
   "\
List of functions to be called to try to inhibit `set-auto-mode'.

Only used by `inhibit-set-auto-mode'.  If one of them returns
non-nil, inhibit `set-auto-mode' and the rest are not called."
   :group 'inhibit-set-auto-mode
   :type 'hook)
;;
(defun inhibit-set-auto-mode--around-advice
     (function &rest ...)
   (if (run-hook-with-args-until-success 'inhibit-set-auto-mode-functions)
       (let (enable-local-variables
             interpreter-mode-alist
             magic-mode-alist
             auto-mode-alist
             magic-fallback-mode-alist)
         (apply function ...))
     (apply function ...)))
;;
;;;###autoload
(define-minor-mode inhibit-set-auto-mode
   "\
Toggle inhibiting `set-auto-mode'.

With a prefix argument ARG, enable the mode if ARG is positive,
and disable it otherwise.  If called from Lisp, enable the mode
if ARG is omitted or nil.

See `inhibit-set-auto-mode-functions'."
   :group 'inhibit-set-auto-mode
   :global t
   (if inhibit-set-auto-mode
       (advice-add #'set-auto-mode
                   :around
                   #'inhibit-set-auto-mode--around-advice
                   '((depth . 100)))
     (advice-remove #'set-auto-mode
                    #'inhibit-set-auto-mode--around-advice)))
;;
(provide 'inhibit-set-auto-mode)

and then configure

(add-hook 'after-init-hook #'inhibit-set-auto-mode)

(defun inhibit-set-auto-mode-line-number-display-limit-function ()
   (and (integerp line-number-display-limit)
        (> (buffer-size) line-number-display-limit)))
(add-hook 'inhibit-set-auto-mode-functions
           #'inhibit-set-auto-mode-line-number-display-limit-function)

Regards,
Alexander