Official comment
Firmare:
1) Make sure the USB drive is formatted to FAT32 with Master Boot Record (MBR).
2) Download the firmware with a browser other than Safari as it can corrupt the file when it automatically unzips the file.
3) Place the unzipped file at the root of the USB drive, making sure not to change the name.
As for rekordbox, disable any functions you're not using (such as Lighting, Lyric, Video, DVS), and if browsing is slow, hide the preview waveform column by right-clicking on the header and unchecking it from the list. Our rekordbox team is working to improve that.