High Speed Hijinks is the fifth and final Challenge Act in Sky Sanctuary Act 2 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the classic era and is playable only as Modern Sonic.
This Act takes place within a heavily modified version of Sky Sanctuary Act 2, where additional enemies, obstacles, and platforms have been placed throughout the Act.
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 Eggrobos and the lasers they shoot, as well as moving platforms.
The player begins on a platform some distance beyond the normal beginning of the Act. There are numerous enemies nearby which shoot twice as quickly as they would normally, and must be carefully avoided or destroyed in order to avoid taking damage. From there, the Act continues much like it would normally, with the player being launched into the first temple section by a Jump Panel before entering a platforming section outside. After navigating through the many enemies, quickly falling platforms, and swiftly spinning iron balls, the player will reach the Goal Ring in front of the next temple.