Sony Xperia Z2 android tablet is an amazing tablet but it does not let you overclock or underclock natively. Fortunately, if you use a custom kernel, you will be able to overclock / underclock with ease. This works only on Xperia Z2 tablet with unlocked bootloader. With this, GPU is overclocked to 657MHz; it idles at 27MHz and add 100MHz step.
How to flash the custom kernel
1. When you are on CM/a CM based Rom: Flash the zip file in recovery
2. When coming from Stock: Flash the boot.img included in your ROMs zip file using fastboot. Flash a CM based Rom then and flash the Kernel zip file again as the Rom overrides the Kernel.
I have installed it on CM12.1 and it runs quickly and smoothly. Read Xperia Z2 tablet review.