The eagerly anticipated iPhone 18 Pro series, featuring models with up to 2TB of storage, offers ample space for high-resolution media. However, an important caveat has emerged regarding the performance of these higher-capacity variants, particularly the 2TB iPhone 18 Pro Max.
Reports suggest that under heavy workloads, such as continuous 4K video recording or large file transfers, these devices may exhibit considerably slower data write speeds. This potential performance trade-off stems from the type of NAND flash memory utilized.
Storage Speed Discrepancy Revealed
Tests conducted by the HOMOLAB channel on Bilibili highlight a significant performance gap. A 1TB iPhone 18 Pro Max, believed to use QLC (Quad-Level Cell) NAND, showed a sharp decline in write speeds once its faster storage cache was depleted. In some scenarios, speeds plummeted to as low as 1.1MB/s, especially when the device was approximately 60% full.
For comparison, a 512GB iPhone 18 Pro, reportedly using TLC (Triple-Level Cell) storage, maintained a notable performance advantage, scoring 38% higher in low-queue-depth mixed workloads and 12% higher in heavier tasks.
QLC vs. TLC NAND Explained
The core difference between QLC and TLC NAND lies in their data density. TLC stores three bits of data per memory cell, while QLC stores four. This higher density allows manufacturers to offer larger storage capacities more cost-effectively, but it can come at the expense of write performance, particularly during continuous data operations.
Understanding the Speed Drop Mechanism
Modern SSDs, including those in iPhones, employ a multi-tier caching system to boost performance. Initially, the iPhone 18 Pro Max utilizes a fast SLC (Single-Level Cell) cache, capable of speeds up to 3,000MB/s. Once this cache is full, a secondary buffer, likely based on TLC technology, takes over, operating at an average of 578MB/s.
The significant speed reduction occurs when both these caches are exhausted, forcing data to be written directly to the slower QLC storage. HOMOLAB measured average speeds of 79.4MB/s at this stage, with intermittent drops to 25.6MB/s. The problem exacerbates as the storage fills; when 60% capacity is reached, the effective SLC cache size reportedly shrinks, leading to direct QLC writes averaging 45MB/s and occasionally falling to 1.1MB/s.
Who Should Be Concerned?
For the average iPhone user, these performance differences are unlikely to be noticeable. Everyday tasks such as web browsing, messaging, casual gaming, and photography typically do not generate sustained heavy write loads that would exhaust the device's caches.
However, professionals and power users who regularly record high-bitrate 4K or ProRes video, transfer exceptionally large files, or perform frequent full device backups and restores, might encounter these slower speeds. These specific usage patterns are where the distinction in NAND technology becomes critical, potentially affecting workflow efficiency.