Ok so I was thinking of the wrong device when I said to disable startup apps.
It turns out that the 77% of "System Idle Process" means it's idle, and it's the percentage of what is NOT being used.
I'm not sure what else would be slowing down your device, but is there anything else on the list that takes a chunk of CPU Usage?
Do you have any apps that constantly run in the background hat may be eating some system resources?