Solder doesn’t increase performance (the memory is soldered to something regardless, either the main board or an expansion board), but shorter physical distances mean lower latency and less power to transmit the same data. LPDDR4/5X are designed to take advantage of this additional efficiency.

It would seem to be rational that the less mass of metal in a connection, the faster that connection will charge or discharge voltage. Physical sockets require a lot more mass just to ensure solid contact.

Well, that too, but that’s not particularly common on laptops or GPUs. Even in Apple silicon it’s not the same die, but it is the same package.

Shorter physical distance means less latency and lower power. Some memory types like LPDDR4X are built with assumptions that only apply to soldered RAM.