High CPU usage with Android emulator (qemu-system-i386.exe)

The cause of the constant CPU usage could be the sound. If you do not need sound in your emulator you can disable it by editing the AVD's config file.

Change/add those two lines

hw.audioInput=no
hw.audioOutput=no

Update: As buncis commented you can also try to disable GPS if not needed:

hw.GPS = no 

On Linux/Mac the file is located at ~/.android/avd/<AVD_Name>.avd/config.ini
On Windows the file is located at C:\Users\<username>\.android\avd\<AVD_Name>.avd\config.ini

On Mac I noticed that the emulator was using over 100% CPU after my laptop went to sleep. Turning the audio off as Benjamin suggested didn't fix that problem.

Restarting the simulated device by long-pressing the power button on the emulator works for me.