With the enchantment refactors in the most recent snapshot at the time of writing, Soul Speed appears to trigger when the player is "unsupported." Cases include swimming, climbing scaffolding, and falling in the air.

To reproduce:

1. enter a save.
2. acquire and equip boots enchanted with soul speed.
3. run or walk while jumping and/or falling off of blocks.
4. notice the momentary speed increase when air is below the player. If FOV effects are enabled, this will be obvious and jarring due to the FOV effect that occurs when speeding up.