It is rumored that the random restart issue plaguing some 64GB and 128GB owners of the iPhone 6 and iPhone 6 Plus are because of the TLC NAND flash chips used by Apple. The company is expected to switch back to MLC based chips to alleviate this issue.

Apple had traditionally used MLC based chips inside its iOS devices, and it was only with the new iPhones that it made the switch to TLC based NANDs. Both types of NAND have their own advantages but TLC based NANDs are usually considered the fastest. Corroborating this fact are the folks over at KBench who ran a test two 64GB iPhone 6 units: one with TLC based NAND and another with MLC based one.

In all the benchmarks ran by them, the MLC NAND equipped iPhone 6 performed consistently faster than the TLC based one. In the Zero filling test, the former averaged a transfer rate of 75MB/sec while the latter averaged at 26MB/sec with a peak transfer rate of 48MB/sec. In the second test, where random data was written to the NANDs — the MLC NAND flash based iPhone 6 reached a peak transfer of 15.7MB/s with a minimum speed of 2.9MB/sec. On the other hand, the TLC NAND flash equipped iPhone 6 only reached a maximum transfer peak of around 2.5MB/sec. The recently released iOS 8.1.1 update should improve the performance of TLC based NANDs to a certain extent and should also fix the random reboot issues being faced by many.

A slow NAND performance can heavily affect the performance of a device, especially while installing apps and doing heavy multitasking. Considering that Apple charges so much for the iPhone 6 and iPhone 6 Plus, it is disappointing to see that the company made the switch to TLC based NAND chips for the devices even though they offer noticeably poor performance. Hopefully, the random reboots bug being faced by some users will force the company to completely transition to MLC based NANDs.