It’s the 16GB. The memory pressure is indeed the issue. I find it hilarious that Apple accidentally inverted their LRU into an MRU and constantly swaps for no reason, yet it’s still so fast that nobody even noticed during QA. Mine has already read/wrote 48TB in the first three weeks; at this rate the SSD will be dead in three years.

(If you haven’t looked into SSDGate yet, be sure to check your smartmon to see whether you’re affected.)