In the video, the free PerformanceTest Mobile [Direct Link] app is used to test the devices, a 32GB iPhone 7 and a 128GB iPhone 7. Both devices have similar read speeds, with the 32GB model reading at 656MB per second and the 128GB model reading at 856MB per second. However, there's a larger discrepancy in the model's write speeds. While the 128GB model writes at 341MB per second, the 32GB model writes at 42MB per second, nearly 8 times slower than the higher capacity model.
Hilsenteger then performed a real-world test more representative of what a user might encounter, using a MacBook and USB cable to transfer movies over to the two iPhone models. The 256GB model wrote the 4.2GB movie in 2 minutes and 34 seconds while the 32GB model wrote the same file in 3 minutes and 40 seconds.
Larger capacity SSDs often perform better than smaller capacity SSDs because its controller has access to more NAND flash memory chips, according to How-To Geek. Manufacturers have to place more NAND chips in higher capacity SSDs, and when they do they place them in parallel to each other. This means that the SSD controller has an easier time accessing more of the memory than a controller in a smaller capacity SSD, which wouldn't have the NAND chips in parallel to each other.