iPhone 18 Pro Max QLC NAND shows significant write speed drops under heavy load

QLC NAND Adoption in iPhone 18 Pro Max

Apple's iPhone 18 Pro series, released last week, incorporates QLC-based NAND for its higher-capacity storage variants. This decision allows for increased storage density, as QLC stores four bits per memory cell compared to three bits for TLC NAND.

Performance Discrepancies Identified

Tests conducted by Bilibili channel Homolab on a 1TB iPhone 18 Pro Max revealed significant performance drops under sustained heavy workloads. While 4K read tests showed similar performance to TLC-based models, the QLC model lagged by 38% in low-queue-depth mixed workloads.

Under sustained write tests, the device utilizes a fast SLC cache (up to 3,000 MB/s) and a secondary TLC-based buffer (averaging 578 MB/s). Once these caches are exhausted, direct writes to the QLC NAND can fall to 79.4 MB/s, occasionally dropping to 25.6 MB/s.

Impact of Storage Fill Level

The performance degradation worsens as storage fills up. When the device reaches 60% capacity, the available SLC cache shrinks from 250GB to 58GB. This reduction also causes the secondary buffer to slow to 396 MB/s, and direct QLC writes average 45 MB/s, with occasional drops to 1.1 MB/s.

Real-World Implications

These benchmark results represent a worst-case scenario and are unlikely to affect typical user activities like web browsing or gaming. However, users engaging in specific demanding tasks, such as 4K or ProRes video recording, large file transfers, or data backup restores, may experience noticeable slowdowns. The findings highlight the performance trade-offs associated with using QLC NAND in high-capacity flagship smartphones.