The storage of the iPhone 18 Pro Max can be forced to run at very slow speeds under artificially extreme, sustained loads. You probably won't ever see this.

The internal storage of a smartphone needs to be fast for most tasks, especially when it comes to storing video footage from the cameras. However, while most brief tasks enjoy this speed, it seems the new iPhone 18 Pro Max may have a limitation under extremely heavy loads.

In an analysis from Bilibili channel Homolab spotted by NotebookCheck on Monday, the type of storage Apple uses in the iPhone 18 Pro Max's 1TB-capacity model can cause problems under tougher loads. In some situations, it can get extremely slow.

This is as-expected. Apple uses quad-level cell (QLC) storage instead of triple-level cell (TLC) flash storage in its higher-capacity models. This was rumored in July.

In Homolab's testing of the QLC-based 1TB and TLC 512GB models of iPhone 18 Pro Max, 4K read tests showed that the two were fairly close, with a gap of about 11% in favor of the QLC storage.

However, under a Queue Depth 1, Thread Count 1 (Q1T1) benchmark, which measures how storage deals with one sequential task in a sustained chain, the 1TB storage speed falls behind.

Under the test, the 512GB model scored 11,285, but the Q1 reached only 8,168. This represents a 38% difference in performance for low queue depths.

There's a gap for other queue depths too, but they are smaller. For Q4T4, the 512GB scored 35,992, about 12% better than the 1TB's 32,219.

This is all reasonable, but the testing discovered that under severely heavy loads, the 1TB storage can crawl at very slow rates.

Under sustained testing, it reached a speed of 79.4MB/s, and even hit 25.6MB/s on occasion. This is normal

Clogging up slows it down

The problem comes down to the use of QLC flash memory for storage over TLC. As the names imply, QLC can store four bits of data per memory cell, while TLC stores three bits. Every time a single bit changes, all four bits need to be rewritten with QLC SSD media.

With the creation of 3D NAND, TLC memory is considered a good choice if you want to balance the speed of accessing stored data with capacity.

There's also durability to consider. Due to writing more bits to each memory cell, QLC is both slower and less durable over time than TLC memory.

To get around these limitations, Apple included a small cache of single-level cell (SLC) SSD media, which handles brief write commands and acts as a buffer with the main storage capacity. This cache is extremely fast, in the region of 3,000MB/s, so long as it has available room in its very small capacity.

Once this is exhausted, there's a second buffer of slower storage, which does the same job. This memory operates at TLC speed levels, around 578MB/s in tests, but it too can be exhausted.

Eventually, if both initial and secondary buffers are full, the data is directly written to the storage. However, this is at the mentioned glacial speed.

The testing also found that the amount of free space on the drive can make things worse, too. With the drive at 60% capacity, the lower amount of free space cuts into the fast cache capacity severely, from about 250GB of SLC cache to nearer 58GB.

This also impacts the TLC buffer, cutting the speed down to 396MB/s. With the buffers exhausted again, the QLC writing speed averages to 45MB/s, and occasionally hit a glacial 1.1MB/s.

Not a problem for consumers

The testing is very much a worst-case scenario situation, pushing the iPhone 18 Pro Max's storage to its extreme limit. Any storage system would suffer under extreme simulated workloads.

It sounds bad, but it's not something the average consumer will ever experience. Under normal loads, or even gaming, the caches are easily capable of handling everything.

To achieve the results in the test, you have to be writing tens of gigabytes of data over time, which is an unrealistic scenario for an iPhone in most cases. Recording high bitrate 4K footage for a very long period of time, meaning 30 minutes at a time or similar, or other large file transfers may be affected, but that is certainly the exception and not normal usage.