Tor client slow to bootstrap using ClientUseIPv4 0

When I run an IPv6-only Tor client using:

tor DataDirectory /tmp/tor.$$ SOCKSPort 0 ClientUseIPv4 0 UseMicrodescriptors 0

It is sometimes very slow to bootstrap.

I wonder if it's a poor quality IPv6 connection, and perhaps an interaction with the directory re-use feature?