This Ableton tutorial walks through the Freeze Track function, a quick way to reduce CPU load on a heavy session. Right-clicking a track (or control-clicking on Mac) and selecting Freeze Track renders all the instruments and effects on that channel down to audio, and the difference in the CPU meter is immediate and dramatic.
Freezing is especially useful when a single channel is running multiple synths or effect chains and your computer starts struggling to keep up. The frozen track stays fully playable in your session, just without the real-time processing overhead.