Low-Level System Optimization
SystemOptions, SetSystemOptions — test and set hundreds of internal kernel options
Options, SetOptions — options for functions, notebooks, and the whole front end
ClearSystemCache — reset the Wolfram System by clearing internal cached results
Update — force internal updating of symbol values
Share — internally share common subexpressions
Operating System Optimization
Run — change process parameters through the operating system
SetEnvironment, GetEnvironment — get and set environment variables