Suddenly the "diagnosticd" daemon rise up to over 200% CPU usage
Solution 1:
I had the same issue and realized that some process (in my case osascript) was flooding the system log with error messages (AEGetDescData of non-desc type 'reco' not a good idea). This apparently led to lots of CPU intensive processing in the Console app (I see that's pretty high up on your process list as well) and the diagnosticd process .
Quitting the Console app solved the issue.
Solution 2:
As a software developer:
Quitting the iOS Simulator resolved the issue.