save-hist-additional-variables never get truncated by save-hist, leading
to a massive cache file and slowdown. Practically, this matters when the
cache file silently grows to 1/2 a GB, and emacs begins randomly freezing.

history-length is supposed to keep a limit on the max history for
save-hist-additional-variables, but it doesn't.

In the savehist-save function:

(dolist (symbol savehist-minibuffer-history-variables)
          (when (and (boundp symbol)
                     (not (memq symbol savehist-ignored-variables)))
            (let ((value (savehist-trim-history (symbol-value symbol)))
      ;;....
  ))))

(dolist (symbol savehist-additional-variables)
  (when (boundp symbol)
    (let ((value (symbol-value symbol)))
    (when (savehist-printable value)
    ;; ...
    ))))

As you can see, the save-hist-trim-history fn is not called in the
second code block.