I began adding new users to my site tonight, all as authors. Is there a limitation to the number of users or how quickly they are added, after adding the 50th user there seems to be a minute or two delay before wordpress will allow me to add the next user.
Wordpress doesn't have any real limitation to that although your
You may also try disabling plugins one by one just in case there is a badly coded WP hook/action.
As a final solution, I would add a huge list of users directly through a SQL query, that way you can add thousands of them in less than a second.