Something on your computer is hogging its processing power.
Before you start typing in Word, open Windows Task Manager via right-clicking on the Task Bar at the foot of your screen and click on the Processes tab. Then maximise the window before minimising it. (You need to have the Task Manager open before the problem occurs, otherwise you might not be able to access it quickly enough).
Type in Word as usual but, as soon as the problem occurs, maximise the Task Manager window and look down the 'CPU' column for any really high figures. (With the exception of 'System Idle Process', everything should be a low number in that column). If anything is unusually high, that program will be the cause of the problem.
If there are no high figures in the CPU column, watch the figures in the 'Mem Usage' column carefully, looking for any figure (even if small) that keeps changing every few seconds. Again, that could indicate the cause of the problem.
If you can identify the program that's hugging resources you'll then have to decide whether you can live without it on your computer. You might need to post here again to identify the actual program though, because the process name listed isn't always easy to link to a particular program. For example, a well-known cause of your problem is 'MsMpEng.exe' hogging resources but that doesn't help if you don't know what it is! (It's actually the bit of Microsoft Security Essentials that regularly updates it. If that was the cause of your own problem, the obvious solution would be to ditch MSE and use a different security system).