iPhone 18 Pro Max 1TB storage may slow dramatically under heavy workloads
The 1TB iPhone 18 Pro Max may not deliver the kind of sustained storage performance some buyers expect from Apple’s highest-capacity flagship. New real-world FIO benchmark testing from the Bilibili channel HOMOLAB suggests that the 1TB model can slow down significantly during long write-heavy tasks, especially once its fast cache is depleted or the phone’s storage begins to fill up.
The issue appears to come down to the type of NAND flash used in the 1TB configuration. While many modern premium smartphones rely on TLC flash storage, which stores three bits of data per memory cell, the 1TB iPhone 18 Pro Max reportedly uses QLC flash, which stores four bits per cell. QLC allows manufacturers to offer more capacity using the same physical space, but it usually comes with trade-offs in speed, consistency, and long-term endurance.
That difference may not matter much during everyday use, but it becomes more important when the phone is pushed through demanding tasks such as recording long high-resolution videos, transferring large files, restoring a full backup, or handling professional media workflows.
HOMOLAB compared the 1TB iPhone 18 Pro Max with a 512GB iPhone 18 Pro using TLC-based storage. In 4K random read testing, the two phones performed fairly close to each other, with the QLC-based 1TB model scoring 46,045 and the TLC model reaching 51,282. That suggests normal read-heavy tasks may feel similar in many situations.
The gap became more noticeable in mixed read and write workloads. At lower queue depths, the TLC-based model was around 38% faster, scoring 11,285 compared with 8,168 on the QLC-equipped 1TB version. Under heavier workloads, the difference narrowed, but the TLC model still stayed ahead, scoring 35,992 versus 32,219.
The biggest concern appeared during sustained write testing. Like many modern devices, the iPhone 18 Pro Max uses a fast SLC cache to handle quick bursts of data. When that cache has available space, write speeds can exceed 3,000 MB/s, which is more than enough for typical smartphone tasks.
However, once the SLC cache runs out, performance begins to fall. The phone then appears to rely on a secondary buffer that behaves more like TLC storage. In HOMOLAB’s testing, this stage averaged around 578 MB/s, though performance varied heavily.
After that secondary buffer was exhausted, the phone was forced to write directly to raw QLC storage. At that point, sustained speeds dropped to an average of just 79.4 MB/s, with the lowest recorded speed falling to 25.6 MB/s. That is an unexpectedly low result for a high-end smartphone, particularly one marketed toward users who may need large amounts of storage for demanding media work.
The slowdown becomes even more severe as the device fills up. When the 1TB iPhone 18 Pro Max was tested at 60% capacity, available cache space dropped sharply. The SLC cache reportedly shrank from roughly 250GB on an empty drive to about 58GB. That means users with a large photo library, lots of apps, downloaded media, or previous video projects may hit the slower write stages much sooner.
At 60% full, the secondary buffer also slowed down, averaging 396 MB/s instead of 578 MB/s. The raw QLC write stage performed even worse, falling to an average of 45 MB/s. In the most extreme case, speeds reportedly dropped as low as 1.1 MB/s.
For casual users, this may never become a serious problem. Messaging, web browsing, social media, app usage, and casual photography usually rely on short bursts of storage activity. In those situations, the fast cache should hide most of the weakness of QLC storage.
The concern is more relevant for people who buy the 1TB iPhone 18 Pro Max specifically because they need the extra space. These are often users who shoot long 4K videos, record in ProRes, move large files, edit media on-device, or keep huge local libraries. Ironically, those are also the scenarios most likely to expose the storage slowdown.
The pricing also makes the situation more controversial. The 1TB iPhone 18 Pro Max carries a substantial storage upgrade cost, yet the reported use of QLC flash means buyers may be getting higher capacity with weaker sustained performance compared with lower-capacity configurations that use TLC storage.
In short, the 1TB iPhone 18 Pro Max should still feel fast in normal daily use, but it may not be the best choice for users who expect consistent high-speed storage during long, demanding workloads. Anyone considering the 1TB model for professional video recording, large file transfers, or heavy content creation may want to pay close attention to real-world storage performance before upgrading.
