I had a conversation with a vendor yesterday. They are interested in including Tor as their "private browsing mode" and basically shipping a re-branded tor browser which lets people toggle the connectivity to the Tor network on and off.
They very much like Tor Browser and would like to ship it to their customer base. Their product is 10-20% of the global market, this is of roughly 2.8 billion global Internet users.
As Tor Browser is open source, they are already working on it. However ,their concern is scaling up to handling some percent of global users with "tor mode" enabled. They're willing to entertain offering their resources to help us solve the scalability challenges of handling hundreds of millions of users and relays on Tor.
As this question keeps popping up by the business world looking at privacy as the next "must have" feature in their products, I'm trying to compile a list of tasks to solve to help us scale. The old 2008 three-year roadmap looks at performance, https://www.torproject.org/press/2008-12-19-roadmap-press-release.html.en
I've been through the specs, https://gitweb.torproject.org/torspec.git/tree/HEAD:/proposals to see if there are proposals for scaling the network or directory authorities. I didn't see anything directly related.
The last research paper I see directly addressing scalability is Torsk (http://www.freehaven.net/anonbib/bibtex.html#ccs09-torsk) or PIR-Tor (http://www.freehaven.net/anonbib/bibtex.html#usenix11-pirtor)
Is there a better list available for someone new to Tor to read up on the scalability challenges?