Speed sketching a house: Just look at the big shapes and concentrate on drawing them on paper. Not each and every window, bar or roof, just the main shapes that you see. After that look at the values - are the colours dark or brighter, based on that choose 3-4 colours and start painting, alternating amongst those colours. Before you realise your speed sketch will be done. Yep that easy. But remember, practice is the key, the more you practice, the less nervous you will feel, it will actually excite you, to test your limits, and kind of go beyond each time :-).