iPhone 18 Pro Max's QLC storage shows severe slowdown under sustained write loads
Independent testing by Bilibili channel Homolab found that the 1TB iPhone 18 Pro Max, which uses QLC NAND, can drop to write speeds as low as 25.6 MB/s after its cache buffers are exhausted. In a low-queue-depth mixed workload, the QLC model scored 38% lower than the TLC-based 512GB iPhone 18 Pro. The higher storage density of QLC enables larger capacities but introduces a notable performance trade-off for heavy data writes.
The performance gap stems from QLC's four-bit-per-cell architecture, which packs more data into the same physical space but writes more slowly than TLC's three-bit design. Apple layers a fast SLC cache over the QLC storage to mask this limitation, but the cache shrinks dramatically as the drive fills—from 250GB when empty to just 58GB at 60% capacity.
Homolab's tests represent worst-case synthetic scenarios; everyday activities like browsing, messaging, and photography remain unaffected. The slowdowns would only surface during demanding tasks such as ProRes video recording, bulk file transfers, or full-device restores, when the phone must write directly to the underlying QLC NAND.
This trade-off could affect professional users who rely on iPhones for heavy media production, such as videographers shooting ProRes or photographers transferring large batches of files. Consumers choosing the 1TB model for future-proofing may find sustained write performance degrades as storage fills, potentially impacting backup workflows. However, most everyday users may never notice the difference, meaning the practical impact likely remains limited to a niche but vocal segment of power users.