Excel High CPU Usage

My laptop sounded like it was working hard so I opened up Task Manager in Windows to see what was going on and, to my surprise, I found that Excel was reporting greater than 50% CPU usage. Permanently it would seem. After a lot of blind alleys I eventually discovered that the zoom setting on the tabs seemed to be the critical factor. At 100% zoom on all 12 tabs the CPU problem persisted. Setting them all to 90% zoom seemed to make the problem marginally worse (approaching 60% CPU usage). Setting them all to 80% zoom seemed to make the problem go away. 

Searching for what might be the issue, I noted that freeze panes was active on one of the sheets such that the scrolling part of the screen was only just visible at 80% zoom. Trial and error determined that the scrolling part of the screen disappeared at 82% zoom and that was the exact zoom level that the CPU usage problem occurred. I removed freeze panes from the sheet and set the ScrollArea in worksheet properties to achieve the same sort of "locked area" effect that freeze panes was used to create and had a usable workbook again.