Users with IPv6 will usually find a small speed increase, a few milliseconds faster for new connections. The difference is greater if the user's ISP has an overloaded CGNAT router.
Not true, in a failover wan scenario new RAs can mess a ton of stuff up on the local network and are not immediate as the local network renumbers and if routes are flapping a total nightmare. Layer in bad ipv6 routing or handling causing timeouts and fallback to ipv4