Ports and DL speed

Hi!
BT uses ports 6881-6999 right?
does that mean that if i open the whole range of ports
i will get faster dl speeds
compared to just opening say like 5 ports eg. 6901 - 6905

another question.
Does one BT dl use one port or many ports?

My comp becomes really laggy if i open up all 6881-6999 ports.
Is this normal? thx!

Not really. You can have as much traffic as you want passig through a single port. The real limit is on your internet connection.