If you design to a socketed DIMM package, you actually have two trace domains, the CPU to the socket, and the socket to the RAM chips. For that to be reliable, you have to adhere to standards for DIMM (or SODIMM) so that multiple manufacturers can build compliant interchangeable parts.
If you solder the chips down to the main board directly, you only have once domain for the traces, CPU directly to RAM. At that point you can tweak the design for several goals, speed, power, etc.. And you can tune the drive strength on the signalling the same.
That is how you end up with more efficient soldered designs over socketed designs.
If you design to a socketed DIMM package, you actually have two trace domains, the CPU to the socket, and the socket to the RAM chips. For that to be reliable, you have to adhere to standards for DIMM (or SODIMM) so that multiple manufacturers can build compliant interchangeable parts.
If you solder the chips down to the main board directly, you only have once domain for the traces, CPU directly to RAM. At that point you can tweak the design for several goals, speed, power, etc.. And you can tune the drive strength on the signalling the same.
That is how you end up with more efficient soldered designs over socketed designs.