Is it safe to turn off Block IPv6 on my Airport Extreme
It has been my experience, that with OS X, enabling IPv6 is not a good idea. I live in the USA and outside of the USA things may be different. The default should be to prefer IPv6 over IPv4. Since most, if not all, of the USA is still using IPv4, one is better of setting your computer to prefer IPv4 over IPv6. With Windows, this can be done by downloading from this site. 

The problem is OS X prefers neither. Instead it tries to determine which is better. The result is you end up waiting. Sometimes this can be a long wait. It is my understanding that if you can configure OS X to prefer IPv4 or IPv6, the operating system will ignore the setting. The only reasonable option is to turn IPv6 off.