I don't know when big issue 90%, not sure number bottom left, user/ idle / system matter and there is the big issue anyway.
Let's just keep this simple. Look at
this Activity Monitor screenshot on interpreting the Activity Monitor CPU cycle information.
- The graph will give you a quick visual of how many of the available CPU cycles are in use
- This will The percentage allocation between System and User will tell you if the problem belongs to the system or a task you have launched as a user.
- IF THERE IS A PROBLEM and only then look at the % in use by individual tasks to identify the culprit.
CONFESSION: I have misinterpreted the Activity Monitor CPU usage percentage for years and only this week learned how it is
correctly interpreted and that was because I was watching both the task percentage and the total percentage data
at the same time. I suspect I am not the only one to make that same error.