Tests show the iPhone 18 Pro Max’s 1TB storage—which uses quad-level cell (QLC) flash—can slow to near-stalemate under heavy, sustained workloads, though most users likely won’t notice.
Storage performance under strain
The iPhone 18 Pro Max’s internal drive generally performs well, even slightly ahead of the 512GB model in some large read tests, thanks to its newer QLC memory. But in a benchmark using Queue Depth 1, Thread Count 1 (Q1T1)—which simulates a single, continuous write task—the 1TB QLC version falls significantly behind the 512GB model using triple-level cell (TLC) flash. The gap is about 38% in this scenario. Under slightly heavier load (Q4T4), the difference shrinks, with the 512GB model still maintaining roughly a 12% lead.
How slow can it go?
Under more severe sustained write loads, the 1TB version’s speed plunges. At peak stress, it clocks in around 79.4 MB/s, sometimes dropping as low as 25.6 MB/s. When the storage is partially full—around 60%—performance deteriorates further. The primary single-level cell (SLC) cache buffer drops from about 250 GB when plenty of free space remains to nearly 58 GB. The backup TLC buffer slows too, from fast speeds down to approximately 396 MB/s. Without those caches available, writing directly to the QLC layer can average just 45 MB/s, occasionally dipping as low as 1.1 MB/s.
QLC, buffers, and the trade-offs
QLC flash stores four bits per cell, which increases capacity but leads to slower write speeds and reduced endurance compared to TLC flash, which holds three bits per cell. To combat this, the 1TB model uses a hybrid caching system. First, a fast but small SLC-level buffer handles short-lived write bursts. Then a larger TLC buffer kicks in. Only once both are filled does the system commit writes to the raw QLC storage—where things slow dramatically.
Why most users won’t notice
The extreme slowdowns revealed in the testing are unlikely to affect regular users. Everyday tasks like app launches, photo takes, and gaming typically stay within the fast-cache territory—and so don’t trigger those dips. The troublesome conditions involve unusually sustained writes: recording high-bitrate 4K video for 30 minutes or more, or transferring large chunks of data nonstop. Such scenarios are rare in typical daily use.
There’s also a lesson in free space: the more storage that’s empty, the more room for fast caching. As capacity fills up, the buffers get stressed earlier, and speed suffers accordingly.
Analysis. What this test reveals is not a flaw so much as a trade-off. By choosing QLC, Apple ups storage capacity in the 1TB model—but sacrifices peak sustained write performance under extreme load. For most users, the design still delivers impressive speed thanks to clever buffering. Still, those who regularly push their phones—videographers, power users—may want to consider TLC-heavy models or those with less occupied storage space. Going forward, expect comparisons of real-world write performance between the Pro Max’s different capacities to become more important as users balance storage size versus consistent speed.
