Yes when it's raining you should never go above the speed limit and you should always slow down and not drive as fast during rain when the roads are wet than you normally would.
Wet roads are more dangerous and your vehicles reaction time is much slower and your vehicle becomes harder to control when it's raining and the roads are wet.
Driving too fast in the rain and when the roads are wet can cause your vehicle to hydroplane which means your wheels and tires leave the roads surface.