Pronghorn antelopes aren’t fast because of any one specific adaptation. Almost everything about them is built for speed.
They have a large windpipe, heart, and lungs compared to their body size. They have long, cushioned, pointed toes which help absorb shock when running at high speeds. They have an extremely light bone structure. They have long limbs. Their digestive tracts are smaller and use less energy when running.
When someone is the best in the world at something, usually they aren’t doing something right. Usually, they are doing everything right.