That is generally the opposite of reality. IPv6 is generally faster than IPv4.
I don't know what "a couple of years ago" is, but all tier-1 ISPs have IPv6 backbones. There should be no reason for your access ISP to have to tunnel anything to get to a tier-1.
Your statement "Most ISPs (including mine) are using 6rd gateways to get their customers onto the IPv6 backbone" is kind of a red flag. If ISPs aren't connected to a backbone, can you even call them an ISP?