are you sure about ipv6??? N900 isn't really IPv6-friendly by default. it is possible to use IPv6 on it after a lot of 'sweat'

So I think it's most likely you have a completely different problem/cause for your slowness, not IPv6.

What is in your resolv.conf files (in /etc and /var/run) ... remove any IPv6 nameservers if you have them. Do you get any IPv6 addresses assigned to your connections?