While building a soul sand bridge on a lava lake using boots with Soul Speed enchantment. If you stand right on the edge of the soul sand block while crouching, you get the slowness of soul sand. It's like the soul speed enchantment is ignored. The same doean't happen with soul soil. So probably something about soul sand not being a full block.

Steps to simulate the problem:

1 - Wear a boot with "Soul Speed" enchantment.
2 - Apply fire resistence effect so you don't get staggered by fire damage.
3 - Find a lava lake and start placing soul sand blocks on the same level as the lava.
4 - You need to be at the very edge of the soul sand block while crouching to simulate the issue.
5 - while trying to walk on the edge, you will be as slow as if you were swiming on lava and also be set on fire.