Output buffering generally will not affect the speed of your web server by any great amount, unless you choose to compress your content - compression takes up a bit of CPU time, which will slow your server down.
However, the amount of bandwidth you use will be cut by about 40% because of the compression, which means your server will spend less time sending data across the network. Your compression mileage may vary - if you have lots of pictures, this figure will be lower; if you are sending lots of XML, this will be much higher.
Copyright ©2015 Paul Hudson. Follow me: @twostraws.