I tried RC3 but still noticed the problem sometimes.
Now I have maybe found the problem, well at least I found one situation where the problem can be reproduced every time, I don't know if there are also other situations where the problem happens. It's quite easy to see CPU go to 100%, just follow these simple steps :
(1) In preferences, select "Check spelling as you type".
(2) Click the icon for 'Add Section' or 'Add Subsection', or do the same from the Section menu.
(3) Watch the CPU jump to 100%, it will stay at 100% permanently - until something is typed in the new section.
I also tested with different dictionaries (and autocorrect dictionaries) but it didn't seem to make a difference.
Well at least I now found a solution, when I create a new section I just have to start typing in it immediately! I don't know if this is the only problem that causes the 100% CPU, but it makes sense because in the past I never had this 100% CPU problem and I wasn't using the auto spelling checker, now recently I started using the auto spelling checker and I guess the new CPU problem started around the same time.
Also please don't forget about the other problem I reported with the spelling suggestions list having hundreds of suggestions causing it to take ages to scroll to the options.
Looking forward to RC4