A couple more questions to more completely define your system.
1. Do you have a graphics card installed and if so, does it share system ram or have dedicated ram?
2. You seem to be using system monitor to get CPU usage data. Can you look at memory usage ,specifically swap, and see if you have a high rate of swap usage? This would indicate a memory shortage and, since swap requires a read/write to hard drive it slows your system down.
A few things you can try right away.
1.Disable visual effects- go to Preferences=>Appearance=>Visual Effects and check "None".
2. Go to Preferences=>Sessions and remove checks on all items you don't need running in the background (bluetooth, mintUpdate, accessability options, etc.). Then go to Administration->Services and do the same( be careful in services that you do not disable anything necessary for proper operation. If in doubt, don't disable it.)
This will not reduce ram and CPU usage by much but any reduction could help.
Registered Linux User 483387