Whenever a player jumps on a moving platforms, I make him a child of this platform, so the character sticks to this platform. However, when I do that, player’s movements are becoming a lot slower. This is only noticeable when a platform’s speed is greater than 0. There is nothing special about this platform, just a simple object with box collider 2D on it with script that makes it move. Do you guys have any idea what is the problem?
Since the character is now a child of the platform and it’s a moving platform this will affect the speed of the character. The platform has its own update method to move it I’m guessing so that is changing the velocity of the object AND!!! all children objects. Basically when your character is on the moving platform and also moving it is also being affected the platforms movement
My suggestion is to not make the character a child of the platform as that doesn’t seem logical to me 
there shouldn’t be a need for that.