Changing boundary condition makes my simulation slow

Hi there,

Am working on microfluidics. I use laminar flow physics to calculate the velocity of my fluid flow. I have used the boundary condition as Velocity and then entered value in Normal inflow velocity(m/s). I used another physics, Transport of Diluted Species, and i selected Model Input as Velocity field.

Now i changed the boundary condition to Laminar flow and i input the flow rate(m3/s). Again i selected Velocity field in Transport of Diluted Species.

But when i try to compute, it is very slow. Even after 10 mins, there is no progress. Am wondering where am going wrong. Could someone help pls.