As of July 2013, currency in circulationthat is, U.S. coins and paper currency in the hands of the publictotaled about $1.2 trillion dollars. The amount of cash in circulation has risen rapidly in recent decades and much of the increase has been caused by demand from abroad.
Nice research. Crypto definitely has a ton of room to grow.
Transaction speeds will need to increase by a lot if it suddenly had billions of users. People can't wait 10 minutes to check out at the grocery store. Look at how pissed people are by chip credit cards, and that's just a few extra seconds.