I discovered another factor in the CPU problem. Working on a note, CPU went up to ~30%, however I had not changed line spacing on it. On looking into it discovered it was caused by a non-functioning link: the kind that gives an error message when you click on it. If I copied and pasted that link into a brand new note, I got the same high CPU %.
As with line spacing, once the note is affected in this way, you can't fix things by removing the link - CPU % will still be high if you type in that note. The only way to get the CPU back to normal and keep the text is to copy and paste it back as plain text. And it has to be all the text: not just the non-functioning link.
(tried to post this a few days ago, Google Groups is in one of its message-eating moods).