This can happen when you're running low on RAM, so if you have a bunch of different apps running. Because SwiftKey uses powerful real-time language modules, you can run into trouble if you also have loads of other apps running.
There were some legacy issues with lag, but we've nailed pretty much all of those in the latest version of SwiftKey. So, I guess the other answer is you may be using an old version.
Is your installation something you bought on the Market, and the most up-to-date version?