My best guess would be that the spelling checker script is the culprit, I've seen performance issues with it before. You could use a custom config file and disable it. If you copy the ckeditor-mojoconfig.js to make your custom config file and then change this to false:
config.scayt_autoStartup = false;
scayt means "spell check as you type". Turning that off may help performance.
Hope that helps,