I am looking for a fast and a diverse library for my game engine network. I have searched for a while, and I decided to use a simple one because of being fast to understand. I saw there are better choices which use multicast and IPv6. I want to know what are the effects on latency between a IPv4 and a non-multicast network for IPv6 with multicast network.
There is no inherent latency difference between any of the layer-3 protocols (IPv4, IPv6, or multicast on either). Multicast requires all the routers in the path to run multicast routing, and that's why you can't multicast on the Internet unless you do it through a tunnel.
You need to be more concerned about the layer-4 protocols. TCP has more overhead than UDP, but it reliable. You may be able to transfer more data in UDP during a specific time period, but if any data are lost en route, UDP does nothing to try to recover from that.
Beyond that, the question ventures into an area which requires opinion-based answers which are off-topic.