My IPv4 ping times to Google and Facebook are 2.6 milliseconds. I suspect any HE tunnel is going to be worse than that (in a statistically significant sense). I don't know that latency would necessarily be perceptible the majority of the time, but I strongly suspect outliers would be worse and more frequent.

Also, I doubt HE's ipv6 tunnel is going to carry anything close to 1Gbps for me for free ;-).