Freezing on world load up

Firstly, when i launch the game from the minecraft launcher about 75% of the time the game freezes on the mojang screen and then says "not responding" when i click on it. if i do manage to open the game fully, whenever i load a world the game freezes again, stuck on the loading world screen. if i click on it then nothing happens, it doesnt say not responding it just stays frozen. Please help