Can you give us more information? Computer specs? OS and what version of Office you are using?

Despite not knowing the above information. My first guess is you are using a computer which is under powered and barely meets the requirements to run whatever version of Office you are using.

You could be running too many programs at once or in the background and another problem could be your computing habits. Are you leaving your computer on for long periods without restarting. If you are not refreshing your memory (RAM) every so often it can lead to a slower computer.

If neither of these result in a solution please respond.

Also next time you post I recommend you submit more information so people on the forums can help you better.