Just got new internet! WHY is it so SLOW?!?

I figure that it would be really fast, but its not. The guy at the orange shop said our phoneline would get about 11-12mb connection, but i tested it and i got about 5mb, its really slow compaired to BT, but they were a rip off and couldnt afford to pay for them. I used to get about 9-10mb with BT and the difference it totally there as youtube loads VERY slow now? Should i complain to orange or will it sort itself out. I've had it for two days now