Very little, IMO. I'm rarely disappointed at whatever I throw at it. I have seen Django apps work correctly before and I would expect libxml and psycopg to work correctly too.
psycopg2cffi is available to provide Postgres support.
Thanks for that. I just ran a quick django shell test, and fetching 50,000 objects took 6.62 seconds in cpython, and 3.62 seconds in pypy. I haven't done a thorough test of the full app yet, but the basics appear to be working just fine.