Emacs is slow, intermittently hangs, freezes when I "come back to it", profiler says `savehist`

Question

When I use Emacs it seems fine mostly, but since recently every couple of minutes it becomes extremely slow for a while. When I change focus to another app, and come back after a while, it is invariably unresponsive for >10 seconds. Emacs is also very slow to boot now.

I ran the profiler and got this result:

       41175  94% - timer-event-handler
       41174  94%  - apply
       41102  94%   - savehist-autosave
       41100  94%    - savehist-save
       23713  54%     - savehist-printable
           4   0%        #<compiled -0x1c1d3aafa883aee7>
           4   0%       #<compiled -0x1c1d3ab2e57b90e7>
          64   0%   + auto-revert-buffers
           8   0%   + show-paren-function
           1   0%  + timer-activate-when-idle
        1859   4% + ...
         386   0% + command-execute
          65   0% + redisplay_internal (C function)
           3   0% + jit-lock--antiblink-post-command
           2   0% + winner-save-old-configurations

But savehist doesn't seem to have changed recently and it never gave me problems prior. What is happening, where do I go from here?

Answer 1

Check your history file: ~/.emacs.d/history. If it is very large (> 10 MB), quit Emacs, then delete it.

Credit to https://reddit.com/r/emacs/comments/5krh9b/long_emacs_pauses_how_to_troubleshoot_from_here/ (/u/UpInHarms)