I'm playing the Thylean Centaur race (from the third-party Odyssey of the Dragonlords module), which has the following trait:
Quadrapedal Stride. Climbing or maneuvering in tight spaces is considered difficult terrain for you.
Since climbing (unless you have a climbing speed) means you move at half speed, would this ability stack with that rule, making it so I move at 1/4th of my speed?