As said before, the basic IPv6 header has a fixed size. So its processing is more easy by nodes and routers, even simplifies the design of semiconductors that have to process it.

On the other hand, its structure is align to 64 bits, so new and future processors (64 bits minimum), can process it in a more efficient way.

But it also has minor fields, and this also points in such efficiency.

At last, in general, and except a pair of exceptions, middle points of the network (routers), only have to process the basic header, while in IPv4 all headers must be processed.