There was zero speed difference or any changes I noticed last I configured my router, which is why it's still disabled. I haven't seen a single good use for enabling it.
Not having to use all sorts of NAT traversal hacks or use relay servers means P2P communications like video calls will be faster, but only if both sides are on IPv6, so yes, it’s a bit of a chicken and egg. Most mobile phones are dual-stack now, and apps like browsers do “happy eyeballs” where they attempt both IPv4 and IPv6 simultaneously and pick the best.
Besides both sides being ipv6, the chat app has to intentionally support some p2p mode, and both sides have to be NAT-free and have firewalls allowing incoming connections.