It doesn't, Apples SSD performance is fine but unremarkable. Their current machines will do around ~6GB/sec read and ~5GB/sec write, which isn't even at the lim

It doesn't, Apples SSD performance is fine but unremarkable. Their current machines will do around ~6GB/sec read and ~5GB/sec write, which isn't even at the limit of socketed PCIe4 NVMe drives, nevermind the bleeding edge PCIe5 drives which can do up to ~14GB/sec read and ~12GB/sec write (albeit with excessive heat and power consumption for a laptop). Soldering the RAM has legitimate performance benefits, but soldering the SSD is just to save space and upsell overpriced upgrades.