I wasn't able to shut the audio off with the config.ini file, but only with the -noaudio cmd line argument to qemu. However, with Android Studio 3.2, you can't add custom arguments to the emulator, so I made this:
In the emulator dir of the Android SDK, in my case ~/Android/Sdk/emulator/
mv emulator emulator.orig
Make a bash script called emulator with the content:
#!/bin/bash
DIR=`dirname "$0"`
$DIR/emulator.orig -noaudio $*
Then:
chmod +x emulator
Now, the argument -noaudio will be added to all invocations of emulator from Android Studio and will save you a lot of CPU cycles.
The cause of the constant CPU usage is 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
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