diagnosticd is a daemon you need to stop/kill with launchctl. The daemon you most likely need to stop has 'SimDevice' in its name. To get the exact name do:
/bin/launchctl list | grep SimDevice
Then with the appropriate name(s) (there could be more than 1) do:
/bin/launchctl stop 'name'
In my case 'name' was: com.apple.CoreSimulator.SimDevice.380EFF06-B636-49CD-851E-5DB4890641AE
If you use Playground a lot and you usually have more than 1 simulator eating your CPU run this script instead:
/bin/launchctl list | grep SimDevice | awk '{print $3}' | xargs -I %s /bin/launchctl stop %s
Oh, and dont forget to file a bug report with apple here: https://bugreport.apple.com/web/
Btw, its kinda fun to see playground/diagnosticd struggle with your code; if you create a bug or some other elaborate syntax, cpu goes up. If you clear it, cpu goes down (just a bit). Almost hilarious how Apple implemented this...
