The performance penalty is that now my packets travel:
Phone -> T-Mobile -> NAT -> T-Mobile -> Internet -> My hosting provider
Whereas with IPv6:
Phone -> T-Mobile -> Internet -> My hosting provider
That NAT is an extra step, and will add a small amount of latency.
Re:iOS Supporting IPv6-Only Networks
by Dylan16807 on Thursday May 05, 2016 @12:58PM
(#11637795)
The NAT, if set up right, would not cause the packets to take a different route.
Changing the packet format requires nanoseconds, no more than normal packet processing.
It's possible to set up anything wrong, but there's nothing about this technology that would introduce latency when done correctly.
While there may be some implementations that have a small performance penalty, certainly not all of them do. The NAT can be done in FPGA or ASIC, and there's not any difficult work being done to translate the DNS response.
* I've been installing NAT64 networks for several years