Selected answers from the Dumb SEO Questions Facebook & G+ community.
Alan Bleiweiss: Depends on server capability more than anything, I would think. I have clients with 20, 000 301s, no problem.
Doc Sheldon: Which has always been one of the major problems I`ve had with IIS... the way it handles things that are so easily managed with htaccess.
Jason Duke: Building on all the comments above, which I agree with BTW, I`d suggest a few things to think about.
One isRewriteMap - It will make it (maybe not much) quicker but it will be a hell of a lot easier to manage - https://httpd.apache.org/docs/current/rewrite/rewritemap.html
https://serverfault.com/questions/414225/best-way-to-handle-thousands-of-permanent-redirects
The second is simply moving from placing lots of redirects from .htaccess, which is a per site easy way to add configuration options to Apache, and putting them direct into the httpd.conf or vhosts.conf or however you have your webserver set up.
The performance boost will be small but noticeable.
A third option is fix the site and inbound links.... of course this is much harder than the band aid of redirects, but with that said and although sorting out external links is a nightmare, it is possible... However, I`m of the view that band aids work well enough :)
How many 301 redirects would you need in your .htaccess before it would start measurably affecting site speed? 
