Soul Speed is an enchantment in Minecraft. You can apply it to boots, and it will allow you to walk more quickly on soul sand or soul soil. Soul sand is a type of sand found in the Nether and will slow down the movement speed of mobs and players who walk on it. Having Soul Speed enchantment on your boots gives you a huge advantage if you lure mobs onto the sand. Soul soil can be found in the Soul Sand Valley, a biome located in the Nether.
You can get the Soul Speed enchantment by bartering with Piglins. Piglins can be found in the Nether and exchange various items with you if you can give them gold ingots. All you need to do is throw the gold ingots near the Piglins. The Piglin will pick it up, then a short time later will give you a random item, which can be the Soul Speed enchantment at a random level. You can also get Iron Boots with a random level fo the Soul Speed enchantment already on them.
You may, if you are very lucky, get a random drop from dead Piglins of the boots with the Soul Speed enchantments, but this is quite rare. The final place you can get Soul Speed from is as a drop from chests in the Bastion Remnants where the Piglins live. You cannot get the Soul Speed enchantment from the enchanting table, and it will not spawn in chests outside of the Nether.