Idle is when your system is just sitting there, or web-browsing, email, etc.
Load is when you're opening large programs, gaming, or doing other very CPU-intensive tasks.
As far as slowing down your system, yes, there is something called 'scaling'. Scaling is what happens when the CPU gets too hot, but before it's auto-shutdown point. In an attempt to run cooler, some CPU's will cut back their clock speed to try and run cooler. When that happens, yeah, performance is affected. Scaling is different than other power-saving features such as EIST, or AMD's version.
In short, unless your performance gets drastically slower, you start getting auto-shutdowns from the heat, or you really hate seeing an Idle temp in the 50's, don't stress out about it - you're good. If you want to bring the temp down for whatever reason, just say the word and we'll be able to offer some pointers.