From Minecraft Parkour Wiki
  • No longer affected by slipperiness (as are all blocks of height > 0.5b)
  • Slowdown now only applies to the inner surface. The 0.3m perimeter around it acts as a normal block.
  • Slowdown applies when the player is standing less than 0.5b above it (includes standing on a slab).
  • If the player manages to get inside a soulsand block, they will find that it has "intermediate floors" that they can stand on. These floors are at heights 0.125, 0.25, 0.375, 0.5, 0.625, and 0.75. The first four are affected by slipperiness.