Apple’s iPhone 18 Pro and iPhone 18 Pro Max models with 1TB and 2TB storage options are reportedly using slower QLC flash memory, while the 256GB and 512GB variants use faster TLC storage. Tests shared by Homolab on BiliBili show significant differences in sustained write speeds between the storage types.
iPhone 18 Pro Storage Options
The iPhone 18 Pro and iPhone 18 Pro Max are available in four storage configurations: 256GB, 512GB, 1TB, and 2TB.
According to testing, the 256GB and 512GB models use TLC flash memory, where each memory cell stores three bits of data. Meanwhile, the higher-end 1TB and 2TB models reportedly use QLC storage, which stores four bits per cell.
QLC storage offers higher capacity at a lower cost but is generally slower compared with TLC memory.
Storage Speed Difference
To improve performance, Apple reportedly uses multiple storage layers in the QLC models. The system first writes data to a faster SLC cache, then moves to TLC cache before reaching the slower QLC storage.
Testing showed that the SLC cache reached write speeds of up to 3,000MB/s. After the cache filled, speeds dropped to around 578MB/s when using TLC cache.
Once both caches were full, the phone switched to QLC storage, where write speeds dropped to around 79.4MB/s, with some measurements falling as low as 25.6MB/s.
Performance Drops as Storage Fills Up
The tests were conducted on devices with empty storage, where the SLC cache was around 250GB. However, the available cache size decreased as storage usage increased.
At around 60% storage usage, the SLC cache dropped to 58GB. At the same point, TLC cache speeds fell to around 396MB/s, while QLC storage performance dropped significantly to around 1.1MB/s during sustained writes.
Random Read Performance Also Affected
The testing also showed differences in random storage performance. The 1TB iPhone 18 Pro Max with QLC storage was around 38% slower in 4K random reads compared with the 512GB TLC model at low queue depths.
Under heavier workloads with multiple processes accessing storage simultaneously, the performance gap reduced to around 12%, but the QLC model remained slower.
iPhone 17 Pro Models Used TLC Storage
Traditionally, higher-capacity flash storage can offer better performance because more memory chips can share workloads. However, this advantage depends on the type of NAND memory being used.
While SLC storage delivers the highest performance, TLC and QLC reduce costs by storing more data per cell, with a trade-off in speed and endurance.
The iPhone 17 Pro and iPhone 17 Pro Max reportedly used TLC storage even in their 1TB and 2TB variants. The reported shift to QLC storage in higher-capacity iPhone 18 Pro models comes as memory prices have increased significantly.
The real-world impact of the storage change remains unclear, as current results are based on synthetic tests rather than everyday usage.
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