The key to painting quickly and “easily” is to not really worry about the end result and to always imagine the big picture. A big stumbling block for people who first starting out painting is to zero-in on one section of the piece (for example, the eyes in a portrait, which are fun to draw), when you should really always be looking at the WHOLE piece. It took me awhile to figure this out too! I’ve learned to always start with a quick sketch(es), then block in big sections of color and shading without worrying about details initially. Think of painting as slowly bringing a photo into focus - once you have the general idea down, the details just follow naturally.
I hope that helped somewhat!