as far as i know, it is a latency issue. the further away from the cpu, the higher the latency for data transfer. sure it is very minimal difference, but at those speeds a couple micro seconds changes the game a lot.
You know the speed of light is about 300 mm per ns, right? And I've read a conservative estimate of the propagation speed of electric signals in copper is about half that. So, each ~150 mm of copper adds only 1 ns. Minimum NVMe read latency figures are well into the range of microseconds. In other words, it's not about latency.
Now, it could relate to cost, although I've read that even PCIe 4.0 boards had to use PCIe retimers. So, if we assume they're using retimers either way, then perhaps they could get a little more distance? I dunno.
there is no other place to put the slot close to the cpu since the other sides are the vrm's, ram slots, power connections and so on.
Yeah, M.2 slots would seem to complicate board layout, a lot. Yet another reason I don't like 'em.
i'd personally like to see them leave a mobo with 2 ram slots (instead of 4 most people don't even populate fully)
I know, right? Especially when Alder Lake had that horrible penalty (reducing from 4800 to 4400) for having a 4-slot board, even when only 2 slots were populated!
Luckily, I think
that issue was resolved in Raptor Lake?
and then add some type of vertical m.2 slot
Some server boards even have PCIe slots practically in the middle of the board, well away from the backplane. I'm not sure quite what that's all about.