First let me say that the dissenting opinion does not originate from me. A company IT guy was installing some software on my MBA, and noticed I had Activity Monitor running. He advised me that this will slow the machine a little. I replied that I'd had a core shutdown and so I was keeping my eye on the cores. "What?" said he. He then proceeded to tell me that a core shutdown is normal. Well, having read a lot of posts here, and posting some myself, I figured he might be nuts, or incredibly ill-informed. But he was quite insistent, and he was definitely a Mac guy, not a Windows guy. At any rate, I kept Activity Monitor on the rest of the day. Some things I observed: *If you display just the bars, and not the running chart, sometimes a bar will transiently go to zero, but then come back on seconds later. I wouldn't consider this to be a real core shutdown, but it could look that way. *Some normal processes do seem to have a normal core shutdown. I found that if I turned on my screensaver (just one of the regular Apple options) for a minute or so, then turn it off, the Activity Monitor shows that only one core had been running while the screensaver was on. *Usually I associate a core shutdown with extremely slow response, jumpy mouse, etc. But there was a time today, just some random configuration of maybe email and Word or something, when a core shutdown. But the mouse response was fine and if Activity Monitor wasn't running, I might not have know a core was out. Nevertheless, I can still find extremely slow response at the same time as a core shutdown. Though now I think there are other factors involved.