Some of your slow-downs might be related to the OS - current MacOSX is 10.4.5. There have been significant changes since 10.2.

To get rid of Fresnel channel: right-click (or option-click) and you'll be presented with the full shader menu, where you can choose none or value, etc.

I believe the boxed figure in the refraction menu is solely to enter your own value, not to reflect the menu; choosing from the menu makes the box irrelevant. Mind you, I think that's true.