The iPhone 18 Pro and iPhone 18 Pro Max offer 1TB and 2TB storage tiers, but testing by Homolab on BiliBili reveals these high-capacity models use slower QLC flash memory instead of the TLC found in 256GB and 512GB variants. QLC stores four bits per cell versus TLC's three, reducing cost but significantly impacting sustained write speeds.

Apple implements a tiered cache: a fast SLC cache (up to 3,000MB/s), a TLC cache (578MB/s), and finally the QLC bulk storage. Once caches fill, QLC writes plummet to 79.4MB/s, dropping to 25.6MB/s. At 60% capacity, the SLC cache shrinks from 250GB to 58GB, and QLC sustained writes collapse to 1.1MB/s. Random 4K reads on the 1TB QLC model were 38% slower than the 512GB TLC model at low queue depths.

Notably, the iPhone 17 Pro and Pro Max used TLC even at 1TB and 2TB. The shift to QLC coincides with rising memory costs. Real-world impact remains unclear, but synthetic benchmarks show a clear performance penalty for the most expensive storage tiers.