You should really avoid calling GetData on any graphics resource… you are literally copying data back from GPU memory to CPU memory. This is not very fast at all.
We could maybe optimize things a bit more there, but DX11 is different from DX9 and it may just never be able to get the same performance as XNA with this. Even then… it will always be slower than already having this data in CPU memory… regardless of XNA or MonoGame.