My computer is running slow, but task manager doesn't show excessive cpu or memory usage. Why?

My computer is running very slow, so I opened up the task manager expecting to see excessive cpu or memory usage. Nothing. So I closed all applications other than task manager. Still slow. Then I opened up the resource monitor, and found the cpu being used almost to the max! What could cause such a discrepancy? Another words, why does taskmgr show barely any cpu usage, while the resource monitor shows that the cpu is overloaded?

EDIT:
Others have pointed out that I incorrectly identified the blue line in the resource monitor as cpu usage. Thank you for that, but I still don't know what to do about my slow computer.