It's not really about performance, it's about signal integrity. AMD said that even with LPCAMM2, you just wouldn't be able to send a clean enough signal from all these different memory channels to the chip, and they insisted on a soldered design because they just couldn't make anything else work.

Technically it's not wrong to say it's about performance in the sense that if you clocked the RAM slow enough you probably could maintain signal integrity, but we're not talking some small hit to performance here, it'd probably be more like a multi-generational drop in memory bandwidth.