When you resize a window in OS X, there's a nice little animation that shows the window expanding. If you don't like this animation (or it's just choppy and slow on your older Mac), OS X Daily shows us how to turn it off and make the resize instant.
All you need to do is open up Terminal (Applications > Utilities), then type this in:
defaults write -g NSWindowResizeTime -float 0.003
Then, relaunch any apps you have running, and the resizing will be instant from then on. Don't forget, if you're on Yosemite, you can get the old resize behaviour back with an Option-Click.