Today with MacOSX (and Windows) you could have multiple applications running multiple processes on the system. If the processes are not active, they are in sleep mode. This means they will not take any CPU resources, and any memory they are using is purged out via virtual memory if they are in the background.
So if possible, if you worry about not having enough CPU cycles for audio work, just quit any applications that might run and cause unnecessary CPU load: Safari, Mail.app and so on.
Some think that Spotlight is stealing cycles when it does an index. The biggest hit is really if there’s a brand new installation, or a big set of new files that Spotlight needs to index or re-index, otherwise Spotlight is not taking that much concerning CPU traffic, so there’s no need to turn it off (there’s a secret way of doing it, but I will not mention it).
Another thing worth disabling, especially with Ableton Live in live situations, is WiFi. Otherwise the WiFi service is constantly checking for networks that could arrive and disappear, sometimes quite frequently in case the network signal is weak.