1:19:35no-defun-allowedI was sent https://www.chosenplaintext.ca/articles/radix-2-51-trick.html, which suggests using 51-bit limbs for bignums would avoid having to move around carries, making addition faster.
1:24:02no-defun-allowedOr rather, it does move around carries, but only does so at the end, allowing additions to be run in parallel on a superscalar processor.