iPhone 18 Pro Max’s 2TB storage comes with a catch: It may have slower transfer speed

Apple’s new iPhone 18 Pro series comes with up to 2TB of storage, giving you plenty of space for high-resolution photos, videos and other large files. But if you are considering one of the higher-storage variants, there could be a performance trade-off that only becomes noticeable under heavy workloads.

Tests carried out by the HOMOLAB channel on Bilibili indicate that the 1TB model of the iPhone 18 Pro Max, allegedly equipped with QLC NAND, experiences a considerable reduction in its write speeds once the faster storage cache has been fully used up. In one of the tests, the speeds dropped to 79.4MB/s, while in another instance they fell dramatically to as low as 1.1MB/s when the device was approximately 60% full. It should be noted, though, that these figures are based on synthetic tests and therefore are not likely to represent typical everyday usage.

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What differences are there between QLC and TLC?

The amount of data that can be stored in each memory cell is what makes the difference: QLC stores four bits per cell whereas TLC stores three bits per cell. The higher density allows manufacturers to provide greater storage capacities, but it can also have an effect on performance when data is being written continuously.

HOMOLAB carried out a comparison between a 1TB iPhone 18 Pro Max based on QLC technology and an iPhone 18 Pro with 512GB using TLC storage. In testing involving 4K reads, both phones produced similar results. Yet, in a low-queue-depth mixed workload, the QLC device achieved a score of 8,168 whereas the TLC device scored 11,285, which meant that the TLC model had a 38 per cent advantage. When the workloads were heavier, the TLC model still held a 12 per cent lead.

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What causes speeds to drop so considerably?

The iPhone 18 Pro Max initially uses an SLC cache, the speed of which can go as high as 3,000MB/s; when this cache is used up, a second buffer based on TLC takes over and operates at an average speed of 578MB/s.

Once both caches have been filled, the data is written directly to the QLC storage. HOMOLAB measured average speeds of 79.4MB/s at this stage, the performance at times falling down to 25.6MB/s.

The problem gets worse as the storage fills. When the capacity reaches 60 per cent, the SLC cache is said to decrease from 250GB to 58GB and direct QLC writes then averaged 45MB/s, dropping as low as 1.1MB/s in the test.

Should you worry?

For the average iPhone user it is very unlikely. Tasks carried out every day such as browsing the web, sending messages, playing games and taking photographs will not reveal this difference. The gap in performance does, however, become important if you regularly record 4K or ProRes video or transfer large files or carry out backup restores.