When walking on the edge of soulsand the slowdown effect will not be applied.  To replicate this you can place a line of soulsand one block above the ground level then walk along the edge.  placing fence gates in a line next to the soulsand helps to align you so you are almost all the way off the soulsand, but still technically on it.  This also effects the Soul Speed enchantment.  If you enter the Soulsand straddling the edge, and never fully enter the block, soul speed will not activate, but if you continue along the edge after activating soul speed, it will not de-activate even when you switch to running along other block edges.  You can replicate this by continuing the above set up with different blocks in place of soulsand.  Furthermore it appears in this set up that the soul speed boots will not loose durability while you are moving along the edge (I tested this in survival).  If you wish for me to provide a world showcasing this, let me know, and I will make something.  Also, I have not tested this in 1.16, as I discovered it while testing the dripstone blocks.