Swiftness Potion

Once you own Hermes Boots or anything built on top of them, this stops raising your top running speed. The potion and the sprint accessory compete for the same ceiling and the accessory wins. That turns it from an early game staple into inventory clutter faster than most players notice.

Asphalt is the exception. Running along an asphalt highway with the buff active is faster than the same run without it, so it is worth drinking before a long crossing or a prepared arena floor. It adds a sliver of horizontal control in midair too, and it layers with a Slice of Cake and with the speed a Sunflower gives you.

Mounts ignore it completely. If your plan for a fight involves riding something, spend that buff slot on anything else.