There are two documented ways to achieve this:

1. Create the environment variable MATHKERNELINIT with value -threadpriority=-1 (the number -1 under Windows corresponds to the priority Below Normal). Under Windows this can be made permanently via System Properties ► Advanced system settings ► Advanced ► Environment Variables. After creating this environment variable all the new MathKernel processes will have the priority Below Normal by default (and in all installed versions of Mathematica!). System reboot is not necessary.

2. Via the Evaluation ► Kernel Configuration Options... dialog create new kernel and in the Advanced Options add the -threadpriority=-1 command line switch, then make this kernel the Default. This will not affect the Notebooks configured to use other kernels.

There are two documented ways to achieve this:

1. Create the environment variable MATHKERNELINIT with value -threadpriority=-1 (the number -1 under Windows corresponds to the priority Below Normal). Under Windows a permanent environment variable can be created via System Properties ► Advanced system settings ► Advanced ► Environment Variables. After creating this environment variable all the new MathKernel processes will have the priority Below Normal by default (and in all installed versions of Mathematica!). System reboot is not necessary.

2. Via the Evaluation ► Kernel Configuration Options... dialog create a new kernel and in the Advanced Options add the -threadpriority=-1 command line switch, then make this kernel the Default Kernel via the Evaluation ► Default Kernel submenu. This will not affect the Notebooks configured to use other kernels.