It turns out there's a nasty little bug in Classic mode that turns your computers into sluggish zombies. You may have experienced this. When you put your computer to sleep with Classic running and later wake it up, you may notice that your computer inexplicably turns slow and dumb. Upon further investigating and opening Activity Monitor, you find that TrueBlueEnvironment is eating all your CPU. Eating it, I tell you! Knowing that TrueBlueEnvironment is the process name for Classic mode, you quit it and all returns to normal and your computer is once again its alive and kicking self.
This is a perfect illustration of why it's essential to have a system monitor like Menumeters in your menu bar. It's just a simple layout of graphs that can get your attention quickly should your CPU go nutso on you. Here it is with a network, CPU and hard drive monitors:
It's a preference pane, and there are several options to make it less (or more) obtrusive. Very simple.
Of course, if you always have your terminal window open, you can achieve something similar by running "top." Then again, if you always have your terminal window open you probably already know all this.