They have to use DDR4-3200 to be able to put the RAM on a stick. Integrated systems can use LPDDR4x-4267. You can't put that on a stick. It's a trade-off. It turns out it takes extra power to drive high-speed signals across long traces with connectors.
I'm fairly sure the power advantage comes from VDDQ being much lower: .6V for LPDDR4x and 1.2V for DDR4.
LPDDR4x is or at least can be 64 bits wide, just like DDR4.
Judging by reviews, the Framework loses 10% of CPU+GPU performance versus reference designs. That could be due to their memory subsystem or thermal design, or both. Ars Technica said the battery life was "mediocre" but I would have gone with "terrible". Compared to the Dell XPS 13 the Framework has only 60% the life.
