There are several process priority classes (base priorities) available in Windows. These base priority classes, combined with individual thread priorities, result in effective thread priorities.
Haphazard use of the Real-Time priority class may result in other threads being deprived of CPU cycles, particularly if the Real-Time process is fully inundating the CPU, causing contention, and/or if its thread priorities are also elevated. Many system and application threads need servicing to enable proper functionality of the system, so should not be entirely starved of CPU cycles.
Note that Process Lasso’s ProBalance algorithm never raises priority classes. Instead, it dynamically lowers the priority classes of problematic background processes. Due to the design of Windows, lowering the priority classes of interfering processes is much more effective than raising the priority class of important processes.