How much is used? Is it more than when playing a song, or less? I seem to be able to render songs ok which crackle in Live, so I'm assuming less.
Rendering is not done in real time unless you have ext instruments or fx. In this mode Live can run its engines at full throttle which should use 100% CPU because there is no need to wait for the audio interface to ask for more data. This also means that there is no need to keep up with the audio interface either, so no clicks or pops due to the engine not providing the interface with data on time.
In other words, in sections of the song where less processing is needed, the engine will speed up to use 100% CPU. In sections where there is more processing, the engine can slow down. It doesn't matter when the bytes are written to disk.