This is a very vague question. What do you mean lagging? Is it actually running, but very slowly? Or does the program stop responding? Does the solver actually start or are you stuck in the definition stage? If you are doing a very complex mesh it would be normal for your computer to slow down as your CPU chews on the problem.
check some of the discussions about this in the flow section, this is a very challenging and time expensive simulation. i'd highly recommend narrowing the scope to maybe the profile you want to use on a wing to start.
but like amit says, it is hard to say whether you're saying the solution is taking a long time or if there is some interface sluggishness or similar