Many mobile ISPs handle v4 via NAT64 or CGNAT. Routers capable of doing those are far more expensive than regular routers, so there tends to be fewer of them. v4 traffic has to travel out of its way to reach one of those routers, whereas v6 traffic can be handed off sooner with a more direct physical path.

It affects anything where latency matters, e.g. from Facebook: "We’ve observed that accessing Facebook can be 10-15 percent faster over IPv6." (https://engineering.fb.com/2015/09/14/networking-traffic/ipv...).