How to improve emacs performance when view large file?

To help with large files, I've installed my own find-file-hook which turns on fundamental mode (avoids font-lock), turns off undo, and makes the buffer read-only just to avoid any accidental changes (making unnecessary backups of large files).

(defun my-find-file-check-make-large-file-read-only-hook ()
  "If a file is over a given size, make the buffer read only."
  (when (> (buffer-size) (* 1024 1024))
    (setq buffer-read-only t)
    (buffer-disable-undo)
    (fundamental-mode)))

(add-hook 'find-file-hook 'my-find-file-check-make-large-file-read-only-hook)

Obviously adjust the threshold value as you see fit.

If you need to work with really large files, you can use the View Large Files package which allows "viewing, editing and searching in large files in chunks." After requireing the package open large files with M-x vlfi.