Directory Opus uses the Windows desktop composition feature to provide transition animations when doing things like changing folder, opening or closing Lister panels, etc. These are purely cosmetic (eye candy!), but because they use the DWM they are hardware accelerated and so won't slow down your computer. You are free to turn them off using the options on this page if you like.
You can configure the animations used for six basic actions. When you open one of the drop-downs and move the mouse over the list of animation types, a small preview is displayed of the animation effect.
Transition animations (and previews) are not available when desktop composition is disabled. They are also prevented when Windows is running in a mixed-DPI mode (different monitors with different DPIs on each) to avoid bugs in Windows when in that mode.