The rate of spreading is sped up as soon as you defeat the Wall of Flesh and enter Hardmode. However, this rate is then decreased once you defeat Plantera. This should stop the biomes from spreading super quickly if you haven’t prepared your world. Hardmode Corruption, Crimson, and Hallow spread is now slowed after killing Plantera instead of any Mechanical Boss.
Whilst Corruption and Crimson spread slowly and only via grass and thorny bushes during pre-Hardmode, they spread very aggressively and to a multitude of different blocks once the Wall of Flesh has been defeated. Defeating Plantera slows the overall spread speed by 50%.
After the Wall of Flesh is defeated in a World, the Corruption begins to spread at twice the normal speed, gains the ability to spread over sand and stone as well as turning mud into Dirt Blocks with corrupted grass, and it becomes able to spread across gaps of “incorruptible” material up to two blocks long.