[2023] Edit: Kith Can't Read, Misreports Bug

Warpath’s movement speed modifier is 20% less movement speed, which is multiplicative with other modifiers. This is different from sources of increased or reduced movement speed, which are all added together before being multiplied by each source of more or less movement speed. So your movement speed while not using Warpath is 1 * (1 + 0.3) = 1.3, and while using Warpath it’s 1 * (1 + 0.6) * (1 - 0.2) = 1.28.

The game guide has a detailed breakdown of how these different modifiers work. You can find it in-game by hitting G and then typing “more” into the game guide’s search box.