I recall a discovery channel (or similar) video on the process of getting a new bill ready for printing. The plates are still carved by hand, the art done by hand, and it takes years to roll out a new bill since they incorporate new anti-counterfeiting technologies, have to deal with new machinery, etc, etc. 5 years to roll out the $20 bill doesn't seem long given all the work that will be going into it.
To people like Munroe, though, who have grown up not knowing what it's like to not have the internet, and instant digital everything, I suppose it seems terribly inefficient and antiquated. I expect he knows all this, though, and simply disagrees with the process.