Question

why drift velocity is in mm/s yet large current is obtained explain.

Answer 1

Drift Velocity is the velocity with which Electrons drift when volatge is applied across a conductor.
Although the drift Velocity is less but due to the presence of a large no. of free electrons , sufficient current is generated.

Answer 2

though the drift velocity is approximately 1 mm/s this means that the speed of the free electrons in the conductor is 1 mm/s.
Now there are many free electrons present evenly in the conductor.As the battery is connected and the circuit is closed thus the electrons are evenly arranged up in the conductor. Thus as the free electrons start to move then the electrons in front of those free electrons get pushed and thus a continuous large current is obtained due to the movement of charge obtained by the free electrons.