Ugh. I think I was having some of [these problems](https://discussions.apple.com/message/19194684#19194684) before upgrading to Mac OS 10.8, but they are definitely happening quite a bit since upgrading. Both Mail and Spotlight seem to be the culprits: two of Apple’s very own applications. softwater on the Apple Discussion boards suggests turning Spotlight off:
sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
And then, later, when there is a fix for the problem, turning Spotlight back on:
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
I hope Apple straightens this out fairly soon. I’m tired of the regular stuttering in my Mac’s responses to inputs.