The Jellyfish 300Mbps clip works fine off eMMC as well. It starts instantly. Although this is quite out of spec: UHD Blu-ray discs only go up to about 120Mbps.
The 300Mbps Jellyfish clip should work for you. It will also test your USB3 drive’s speed. Let me know if you need Sync Playback to Display for that. The clip has no sound.
Sam