You wont have much performace loss if you install your apps to the SD card. just a little longer load time as the OS has to load before the SD card can be read. Class 2 is the slowest SD card speed. Class 2 has a write speed of about 2 Mb/s and is suitable for standard definition video recording. Class 4 and Class 6 have write speeds of about 4 Mb/s and 6 Mb/s, respectively, and can be used for HD video recording. These three classes of SD cards are designed to operate under the normal bus interface. Class 10 SD cards have write speeds of 10 Mb/s and up and are designed to operate under the high speed bus interface. but that is mainly for used for video capture standards.