Hey there! I'm happy to help!
Imagine if the Earth spun really fast. It would be day for like 2 seconds and then be night for 2 seconds and it would go over and over again. You would probably get pretty dizzy. If the Earth spun quickly, the day would be fast and short.
Now imagine if it were slower. The day would definitely be much longer if you think about it in terms of the example above. If the Earth is moving slowly you'll be in the Sun for longer and in the night for longer. The length of the day would decrease.
Have a wonderful day! :D