Make sure Windows is using all of your RAM. Dolphin may be trying to use the lower end emulator. Many Mac models come with 2 video cards standard, one is a low-end Intel integrated graphics for everyday use and the other is a high-end card for scientific applications, gaming, video/graphics editing and programs where the programmer does't realize they are using a GPU filter instead of a CPU filter.
Make sure you have the video card drivers up to date. There should also be a Boot Camp manager in Windows you can use to see if your drivers are up to date on that end. In Mac OS X, you can go to Apple Menu > Software Update… to update your Mac. Make sure Boot Camp and it's drivers are up to date. There are several things that can impact the performance of your computer under Boot Camp.
