Dolphin seems to run NSMB Wii more slowly with OpenGL than DX9

[RedXelyx]

I was wondering why when I run New Super Mario Bros. Wii using OpenGL it seems to run slower than when I run it with DirectX 9. I am using the official release 4.0.2 of the 64-bit version of Dolphin; my computer's specs are
[color=#000000]Operating System:
Windows 7 x64
Processor/CPU:
AMD 8350 4Ghz
Video Card/GPU:
AMD Radeon HD 7870
Memory/RAM:
Corsair Vengeance 16 GB[/color]

If anyone can tell me anything relating to this issue, I would greatly appreciate it; thanks, in advance!