FCK is all client-side. It's javascript, and javascript is notoriously buggy and loves to hog memory.
Each time you make an edit to a page within the editor textarea it needs to write it to memory for Undo purposes. The trick/solution to avoid problems seems to be to Apply changes as you make them. Not such a big issue.
Or, as others suggest, we could always try another WYSIWYG. I used to use TineCME. The only reason I changed was because FCK became the default editor and was getting a lot more development time. That now seems to have changed and Tiny has all the CMS buttons that FCK has.
elijahlofgren has also done the same thing for Xinha editor, which is a truly wondrous app when it works.

See the projects page for more info.
IMHO I think that the dev team should pick one editor and then continue to purpose-build it especially for CMSMS. It would be better for everyone in the long run. [Hear that Ted?

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