To test the speeds of the 256GB M.2 SSD that ships with the ZenBook, we ran CrystalDiskMark 6.0.0.
The Micron 1100 M.2 drive only uses a SATA interface, so read and write speeds are held back to 523.1MB/sec and 445.8MB/sec. This is decent enough performance for SATA, but NVMe would have been five or six times quicker.
The UX331UN has USB 3.1 Gen 1 ports. We plugged a 32GB SanDisk Ultra USB 3.0 thumb drive into the one on the left next to the USB C port.
The read performance of 126.1MB/sec is about what we expect for this thumb drive, whilst the write speed of 31.28MB/sec is relatively quick.