The font panel generates preview samples for all fonts selected to view. If you have many fonts added in Suitcase and you have your Extensis Font Panel set to display ‘Suitcase Fusion Fonts’ that means that on first launch of Illustrator and the panel, it will be off generating a sample preview for every single font in your collection. For some customers that can mean over 10,000+ preview samples being generated on the fly, thus the CPU usage you see whilst this is ongoing. Once generated it will settle down and remain cached for that session.
If your machine is having problems coping with generating so many previews, you can use Font Digests so that instead of trying to display all ‘Suitcase Fusion Fonts’ at once in the panel, you can choose an individual library or set to display instead, based on what you’re working on in Illustrator at any given time. Thus managing your fonts in Suitcase first, to organise into smaller groupings of fonts (libraries and sets) that you’re likely to want to use in your application, can enable you to work more efficiently, and not be generating so many previews at once in the font panel.