Q:

Elfeed: mark all messages as read

Is it possible to mark all unread messages as read with a single key?  r marks only the selected message as read. 

A:

You can select the whole buffer and mark everything as read using the r default binding in elfeed-search-mode-map. 
You can also write a little function like below:

(defun elfeed-mark-all-as-read ()
      (interactive)
      (mark-whole-buffer)
      (elfeed-search-untag-all-unread))

and bind that to R in elfeed-search-mode-map if you wish. 

(define-key elfeed-search-mode-map (kbd "R") 'elfeed-mark-all-as-read)

Note that at the time of writing this, this mapping was unused in elfeed-search-mode-map.