These games were designed for older processors, and don't have any code written specially for the G4's. System Extensions Current games use the operating system to do certain things. For example, reading files, moving memory, displaying pictures and movies. When the code behind these System Extensions is re-written to make better use of G4 features, such as the Velocity Engine, then they will be faster. The more the games use these extensions, the more they will be improved.
Some games use extensions more than others.
It is quite common to actually avoid using extensions in favor of custom game code, such as memory move or screen blitting code. Such code will not be sped up by faster system extensions. If these extensions are made faster, games will go faster too. Some games can have Existing Pieces re-written with G4 features in mind. 3D math can be speeded up by changing code to use the Velocity Engine.