Benchmark for the second patch:

  (setq v (with-current-buffer (find-file-noselect
                                "~/emacs/test/src/json-tests.el")
            (vector (buffer-substring-no-properties (point-min)
                                                    (point-max)))))

    (progn (garbage-collect)
           (benchmark-run-compiled 1000
             (json-serialize v)))
           
Results for the second patch: 0.7815s applied vs. 0.7962s without.