High Speed Hijinks is the fourth Challenge Act of Seaside Hill Act 1 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the Dreamcast era and is playable only as Classic Sonic.
This Act takes place within a heavily modified version of Seaside Hill Act 1, with additional enemies and obstacles placed throughout the stage.
As the name would imply, during this Challenge Act, everything (excluding Sonic himself) moves at double their normal speed. The increase in speed is meant to make it harder for the player to navigate through the given area to the goal. This includes enemies such as the Egg Pawns and the bullets they shoot, as well as moving platforms.
The Act begins inside the temple, where the player must go through a short underwater section. Once they leave, they will go through the outer areas of the temple, where they must platform across quickly moving pillars and avoid being hit by enemies. The player can find the Goal Plate at the end of the next platforming section.