adding offload pragma , performance drops

Hello ,

I am running a code in openMP which is like this:

#pragma omp parallel for default( none ) shared( X , Y ,V ,V ,H , W ,N ) private ( i,x,y ,Kx,Ky,initD ,T )

		for ( y = 0; y < H; y++ )
		{
			for ( x = 0; x < W; x++ )
			{

				initD = aValue;
				for ( i = 0; i < N; i++ )
				{
					.....Kx,Ky...
                                        ...X,Y..
				}

				V[ x + y * Width ] = T;

			}

		} 

Now , I want to run it on mic card , so when I just add the line:

#pragma offload target (mic) in ( X:length( W ) ) in ( Y:length( H ) ) out ( V:length( W * H) )

 the performance drops drammatically!

What should I pay attention to?What changes do I have to make?

Thank you!