There is no such thing as a normal usage, if you are using 100% of your CPU (which you are not) then your CPU is too weak to run the game (your CPU is absolutely fine), there are many reasons why you can get fps drops but I doubt that your CPU is the issue (I'm just assuming that you are doubting in your CPU performance since you posted the picture of it's performance). There are many reasons why the FPS drops may occur but I doubt that your CPU is the one at fault. The graphics card might be, but I highly doubt (CS:GO requires strong CPU and weak graphics card). My friends usually clean their PC's hard disks when they get lower FPS than they get usually. Also you might want to blow out the dust from your PC, that might cause the FPS drops aswell, if they started to occur recentlly.