iPhone 18 Pro Max 1 TB memory speed may drop to 1.1 MB/s
The iPhone 18 Pro Max with 1 TB storage revealed Kioxia BiCS8Q QLC memory, while the iPhone 18 Pro with 512 GB uses faster TLC NAND.
QLC stores four bits per cell versus three in TLC. This increases storage density but reduces sustained write speed after cache exhaustion and typically lowers the rewrite endurance.
Write speed drops from 3256 to 25.6 MB/s
In the HOMOLAB random read test with 4 KB blocks, the iPhone 18 Pro with TLC was approximately 11.4% faster. In mixed read and write operations, the advantage ranged from 12% to 38%.
During prolonged sequential writes, the difference becomes more noticeable. While the dynamic SLC cache is active, speeds reach 3256 MB/s. Once it fills up, the drive switches to pTLC mode with an average speed of around 578 MB/s.
Then direct recording begins in QLC: average speed drops to 79.4 MB/s, and minimum speed falls to 25.6 MB/s. In this mode, the storage device may prove slower than some budget microSD cards.
A filled drive works even slower
As the storage fills up, the dynamic cache shrinks. On an almost empty iPhone 18 Pro Max, about 250 GB was allocated for it, but when filled to approximately 60%, only 58 GB remained.
The average speed in pTLC mode dropped from 578 to 396 MB/s, while direct write to QLC fell to approximately 45 MB/s. The minimum result was 1.14 MB/s.
When a speed drop can be noticed
In short operations, the difference is usually hidden behind fast caching. Limitations appear during long continuous writes of large data volumes, for example:
- high-bitrate video recording, including ProRes;
- copying large files and archives;
- restoring a large volume of data from a backup.
HOMOLAB results apply to a specific 1 TB iPhone 18 Pro Max. Apple does not disclose the NAND type across different configurations, and third-party data suggest that both QLC and TLC chips may be used in 1 TB models. Therefore, the results cannot be generalized to all 1 TB iPhone 18 Pro Max units.
For working with video and large files, this is significant: speeds exceeding 3000 MB/s are maintained only until the fast cache is filled.
