Low Quality Preview reduces the resolution of the preview, without affecting the final exported video. This setting can be used to reduce preview lag on a device.
Note that this will not reduce all types of lag, only lag caused by peaks in processing usage due to having a lot of effects (such as blur) to process on screen at the same time. It will not affect export, including errors in any way. Note that this option applies only to Android devices (iPhone and iPad devices are optimized so this is not needed).