Determine cpu usage in safari tab
Chrome will do this. 

I know your question is about Safari but bear with me. Chrome works slightly differenty from most web browsers in that each tab is created as a separate process rather than just as separate threads. This means that if a tab starts misbehaving or crashes then it won't bring down the whole application

This also means that you can bring up the Chrome task manager and it will tell you about the CPU and memory usage for each time.

With Safari though.... I don't think this is possible.