When I hear people talk about five minutes of warm ups/stretching, it makes me nervous. OK if you do your own thing before class, but only five minutes total? What do you stretch, just your legs? Just your wrists and shoulders?
The key to a efficient warmup, IMO, is to utilize full-body movements. There is a 2-second sequence that I practice before every class, which opens the ankles, knees, hips, back, shoulders, elbows, and wrists. It also improves circulation, functional coordination and balance, e.g. speed and power. It's a dynamite warmup, even @ 2 minutes.
Then I might spend 2 minutes targeting the neck and 2 minutes on the legs. This is my insurance, in case of a bad fall.