CPU usage will spike when speech starts, that is when the text is being synthesized into audio, the synthesis happens faster than the actual speech (otherwise you would get pauses after every sentence), so once all the text for the current file is synthesized it will idle again.
I cannot reproduce the last word not being highlighted from sentences. If its happening on a particular book open a bug report and attach the book.
Added a button to download/remove voices:
https://github.com/kovidgoyal/calibr...1aacbba0a7f8e0