Android ads slow performance very much
Google AdView has WebView with a lot of animation inside. It will heats up all mobile CPU.
Summing up it's normal. I have the same situation in my applications. AdView take 30% of CPU. I tried to find any solution but it's known issue. Below links can help you optimize your banners but you have to know that, it will always take some of CPU.
stackoverflow - admob banners cause high cpu usage
stackoverflow - webviewcorethread used by admob adview is using high cpu
stackoverflow - android admob consuming more cpu
pzoleeblogen - how to solve adview cpu consuming
Please vote up my answer if I help you. Have fun.