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>Given this architecture, how would read speeds drop by 35% from one model to the next? I'm not sure--my first guess was that fewer NAND chips were being used, but teardowns show that both 2018 and 2019 models were using two chips. So same controller, same number of chips. Maybe the NAND is just slower? Or the T2 has less RAM, so it can cache less?

It is very common, if not the norm to have SSD slower at lowest end capacity. Having 2 Chip shown in the tear down doesn't mean it has the same channels inside those package. My guess is that it is Dual Channel per package, and it is only running at 3 Channels on the 128GB SSD.