What would happen if the speed of Earths rotation was decreased?
If You are talking about just spinning we would have longer days if the speed of our rotation around the sun slowed down we would have longer years
"Earth stopped spinning suddenly, the atmosphere would still be in motion with the Earth's original 1100 mile per hour rotation speed at the equator. All of the land masses would be scoured clean of anything not attached to bedrock."(this is from google)