The Great Reading Adventure should provide little lag to your users. The database is relatively lightweight, but of course, the speed does depend on how you’re planning to host the program. Could you give us a little more information on that?
Just for your information, the Maricopa County Library District hosted the program (using our internal server) for somewhere around 70,000 participants in 2014 and had no issues. We’d be happy to give you more details if needed.