(if the author is reading this) - what were your interest in using python? Do you have any concern for it's performance/scalability as the service grows?
I've been using Python in enterprise-grade, large-scale production systems since the early 2000s. And it works, well. Most python libraries where performance matters have C under the hood. E.g. numpy matrix manipulation. Parallel computing got way better with the async support.
As for BigchainDB itself: it wraps RethinkDB (and soon MongoDB) which is where much of the compute intensive stuff happens. Also, we leverage these fast Python+C libraries, and parallelization. If/when needed, we can always convert some of our core stuff to C or another fast language.