OP, what you've described in the subject is not normal in my experience, however, CPU usage does spike when work is performed in the Page Layout View mode, much less so when working in Normal View mode. If you're using the former View mode, I suggest a switch to Normal View.
I'd read bug reports about this issue as early as last summer. I work in the Windows version of Excel, it too will occasionally peg a GPU or CPU in Page Layout View - I generally only work in Normal View.