XDA has released a custom firmware for the Samsung Galaxy Tab Pro 8.4 tablet. This firmware allows you to easily overclock or underclock the processor as well as GPU. The only pre requirement is that you should have installed Tab 8.4 TWRP before flashing the custom kernel which should not take more than 5 minutes.
Download the kernel from here and flashing it using TWRP. Put it somewhere on your tablet and boot into recovery to flash it. Once you have flashed the kernel, simply install Synapse from play store to start underclocking or overclocking the CPU or GPU of Samsung Galaxy Tab 8.4 tablet. Here is one recommended synapse profile so that you do not crash your Tab 8.4 tablet.
We also suggest you to disable MP-Decision, enable intelli-plug, enable intelli-thermal and set to restore kernel settings on boot.