Apple released the iPhone 18 Pro and iPhone 18 Pro Max in four storage capacities: 256GB, 512GB, 1TB, and 2TB. However, tests revealed a significant difference between these models: the higher-capacity versions use slower QLC memory instead of the TLC memory used in the lower-capacity versions.

Tests conducted by Homolab on two iPhone 18 Pro Max models, one with 512GB and the other with 1TB, showed that Apple uses TLC memory in the 256GB and 512GB versions, while relying on QLC memory in the 1TB and 2TB versions. QLC stores four bits per cell, allowing for larger capacities at a lower cost, but this comes at the expense of write speed.

To mitigate this slowdown, Apple uses layers of cache memory, starting with the faster SLC cache, followed by TLC, and then the QLC cache.

SLC cache write speeds reach approximately 3,000 MB/s, but when it's full, the phone switches to TLC cache, which registers around 578 MB/s. Once both SLC and TLC are full, the phone begins writing directly to QLC, where the speed drops to 79.4 MB/s, and in some cases, can even fall as low as 25.6 MB/s.