My Ubuntu 12.04 LTS with gnome-classic shows a large CPU usage after I highlight any LibreOffice app in gnome menu (Apps->Office-> *).. only on LibreOffice.
What is the reason?
The reason is that Libre Office is quite some ressource intensive. If you want a light system I recommend the office software Lubuntu uses: Spreadsheet, Wordprocessor, Notes and maybe planning calendar.
Go into Libreoffice settings and change Memory > Undo > Number of Steps to 10... Also change View > Graphics output > Turn ON Use hardware Acceleration, Turn OFF Use Antialiasing (looks jaggy but speeds up page drawing)... Hope this helps a little...
Also, turning OFF automatic spell-checking and grammar checking will improve performance...