SD card reading speeds vary depending upon which class of SD card that you have chosen. However, many have experienced disappointment when comparing the read speed on our PCs to that on our Android devices. XDA forum member brainmaster states that it doesn’t matter if you have a fast 10 Class SD Card – it’s all in the Cache.
The Cache size for reading from SD card on your Android device is set to 128KB, on some ROMs even to 4KB!
To check for yourself you will need to navigate to /sys/devices/virtual/bdi/179:0/read_ahead_kb. This can be changed manually but will not persisit on reboot. In order to make the change persist, it needs to be done by loading the script on startup through the init.d. Alternatively, the XDA member has provided the CWM.zip files to make thing easier.
To find out the best Cache size for your SD card and the best method to change this for you, head on over to the modification thread._________