Originally Posted by Official Chankast Manual
The "CPU 100" displays the current speed of the SH4, the main CPU in the dreamcast console. Since Chankast v0.2, you have the possibility to overclock and underclock the CPU to affect the emulation speed during gameplay with the following keys:
+ Overclock the SH4
- Undercolock the SH4
The clock speed ranges from 50% up to any value you can imagine. If you have a slow PC, you might gain some speed if you underclock the CPU slightly. Faster PCs can get more FPS than the actual dreamcast by increasing the clock rate - useful by getting past slowdowns or other stuff.