Future Audio Workshop has released the 1.0.2 update to their fantastic Circle Software Synthesizer. You can download it here. According to FAW:
In this update we have included a CPU economy mode in the setting pane. This allows Circle to use less CPU for a minimal reduction in audio quality.
Behind the scenes, there have been some updates to the OSC messaging and a bug related to the mouth filter clicking during modulation has also be fixed.
The CPU economy mode is off by default. To enable it, click the "Settings" tab at the bottom, then click the "CPU econ. mode" switch to set it ton "on" as shown in the image above. Note, don't set the mode till after you've selected the patch you want to use as changing to another patch seems to turn the setting off.
Another way to reduce CPU load is to freeze tracks in your DAW host software so that the clip in question is rendered to the hard disc prior to playback. Note not all DAW's support this but off the top of my head Ableton Live, Sonar, Cubase, and Logic do.