Hrvoje Niksic <hniksic@srce.hr> answers:
   Since the current Emacsen do not support base64 in native "C" code,
   Pterodactyl Gnus implements base64 decoding and encoding in Lisp. This
   is inherently slow because base64 operations consists of examining
   each character in turn, and doing arithmetics on them -- repeated for
   thousands of characters. However, Gnus allows you to use an external
   base64 decoder, if you have it.
   
   For example, if you have Nathaniel Borenstein's "metamail" package,
   here is how to tell Gnus to use its `mimencode' program to decode
   base64:
   
(setq base64-decoder-program "mimencode"
      base64-decoder-switches '("-u"))

(setq base64-encoder-program "mimencode")

   Note that, beginning with XEmacs 21.2 and GNU Emacs 20.4, the popular
   Emacsen will support base64 natively, which will be faster than even
   the external encoders, making above hack obsolete.
   
   Also note that uudecoding, as performed by mm-uu, is also done in Lisp
   by default. You can make Gnus use the external Unix decoder like this:
   
(setq mm-uu-decode-function 'uudecode-decode-region-external)