Sorry, what? Citation please. The speed lost moving a DIMM from a 3cm path-to-CPU to a removable DIMM slot 6cm away is infinitesimal. You would not notice it.
For laptops, soldered vs socketed DRAM is mostly about power, not performance. But it is true that soldering DRAM makes it possible to reach higher bus speeds than are practical to achieve through a DIMM slot; this is why GPUs don't have upgradable RAM.
Laptops that have soldered RAM (including Apple) do not come with overclocked RAM though. I don't even know of any system with soldered RAM that would let you try pushing the speeds in practice, so this advantage is never leveraged.
But we do know how far DIMMs can go. Mainboard memory trace layouts have gotten ridiculously optimized. DDR4-5200 is a totally "normal" XMP spec now, there are even 5400 kits out there I think.
> Laptops that have soldered RAM (including Apple) do not come with overclocked RAM though. I don't even know of any system with soldered RAM that would let you try pushing the speeds in practice, so this advantage is never leveraged.
This is unsurprising given what I already pointed out about power being more important than maximizing performance. Sure, you can overclock the memory controller on a desktop platform and with sufficiently expensive DIMMs and probably a bit of an overvolt for the memory controller/uncore you can reach higher bus speeds than any laptop CPU (though still not anywhere near as high as GDDR used with high-end mobile GPUs).
But take a look at the fastest memory speeds available through SODIMM slots vs the speed of LPDDR4x supported by the same CPUs. Within the constraints of laptop power levels and using standard grade memory parts rather than requiring premium binned chips, soldering currently corresponds to a 33% performance advantage.
Who is saying the soldered ram is never overclocked? You cant overclock changeable ram by default because you don‘t know what the customer will use. But by soldering the ram you know you will use very specific parts, so you could (maybe they do?) tune the settings because it does not have to work with thousands of combinations.
The speed lost due to distance would be infinitesimal, but is that the only speed loss there would be? I'd expect that parasitic inductance and parasitic capacitance would be higher in a socketed system which would impose speed limits.
Channel != DIMM. Many small channels is just how LPDDR works, as opposed to DDR which has one big chungus channel. Well DDR5 has now split it in two. The total number of traces is the same.