It’s a physiological fact that it takes (usually) at least 15 to 20 minutes for your muscles to properly warm up.
The difference, is your body won’t start burning fat until your muscles are warmed up. This is because you have to burn all your extra sugar and carbs first that your body normally uses for energy.
So, take a light jog, or even a walk for 20 minutes or so before you start pumping iron. It’ll really increase your results and make every workout easier!