Minecraft wiki entry for Elytra, associated text: Slow Falling potions greatly decrease your vertical (fall) speed, which also greatly decreases your horizontal speed (clearly, it depends on the vertical fall rate, as one might expect from flight modeling). The net result is that you fly extremely slowly - but your Elytra still takes durability damage at the usual rate (1 durability per second without Unbreaking, etc.). Due to this combination, Elytra plus Slow Falling is largely a novelty that most players won't use. The Feather Falling enchantment does not affect Elytra speed (but can help protect you if you land hard).