I've found that when the power states of individual effects in the rack are toggled off (green buttons), they are still processed. For example, real time displays on the plugins show active processing, indicating that toggling power state to off simply bypasses their outputs. As a result, the cpu load is as it would be if they were toggled on.
This is a problem with effects that are not meant to be used in real time and interfere with playback, especially if they need to be in the middle of the set. Clean, non-choppy playback cannot be restored by toggling their power states. In order to audition, the effects presenting a load must be entirely removed from the rack, closing them and losing their settings and data such as noise prints. If the modules themselves have their own, internal bypass toggles, the problem may be solved that way, but not all do.
There are two methods of addressing this in Audition 4.
1. In Preferences > Effects, uncheck the box labeled "Process bypassed effects during playback"
2. With the "fx" track panel mode selected in Multitrack, or in the Effects Rack, click the "lightning bolt" icon which will enable track pre-render, which performs a background render of the track and all effects. This process is dynamic, so as you make additional adjustments to the track or effects parameters, it will re-render the track.
The behavior of processing effects that are powered-off stems from the common request for users to be able to power effects on and off to listen to the difference. For high-CPU effects, or effects with long initial delays, this ensures the properly processed audio is heard when powering the effect on during playback. If this isn't a crucial need for your workflow, either of the two options above should help to reduce the processing overhead you're experiencing.