The slowness effect lvl 5 or above makes the player walk slower on soul sand than on slime. The walking speed on slime is (roughly) the same as walking on any other solid block, while it slows the player more than soul sand if the slowness level is lower (or without the effect).

The honey block also acts like the soul sand, and has the same speed (which, again, is supposed to be faster than the slime).

Moving on the slime block is slower with lvl 5 than with lvl 4, but the speed difference doesn't seem right compared to the soul sand.