When you have less time, concentrate on the big shapes, the main values (light or dark), the main colours you wish to add and then just keep alternating amongst those colours. Before you start analysing the painting, you should be able to finish it. Its almost like a 100 m sprint. In this case, I had exactly 8 minutes waiting time before the bus for home would arrive, and I had a complicated city gate with a detailed black clock, red roof and atleast 20 windows staring at me!