Among many of the changes to the Mail application that Lion brought was an animation when you reply or create a new mail window. It’s a nice touch, but on an older Mac such as my original Mac Pro, it becomes a slow, annoying, and frivolous feature. Here’s how you can turn it off.
Fire up the Terminal and copy/paste the following text (and hit return):
defaults write com.apple.Mail DisableReplyAnimations -bool YES
This will turn off the animation when opening new windows in Mail. If you ever want it back, simply replace YES at the end with NO.