Thank you so much for this. This immediately replaced my usage of org-velocity. I used this function to convert my (10,000 line) velocity.org file into separate files for Deft:

  (defun generate-files-from-velocity (buffername)
    (save-excursion
      (switch-to-buffer buffername)
      (org-map-entries (lambda ()
                         (progn
                           (org-mark-subtree)
                           (if (and (= 1 (org-outline-level)) (org-export-get-title-from-subtree))
                               (let* ((title (downcase (org-export-get-title-from-subtree)))
                                      (filename (replace-regexp-in-string " " "-" (concat title ".org"))))
                                 (append-to-file (region-beginning) (region-end) (concat "~/Documents/Org/Notes/" filename)))))))))
  
  (generate-files-from-velocity "velocity.org")