Depending on if I am starting a workout or just moving to a different body part the amount of weight I use for a warm up will alter
Bench for example I would normally start with one or two sets of 20reps @ 70kg
then go to 110kg for 12 or so reps then increasing weight and dropping reps maxing out between 140kg and 160kg depending on reps and how hard I am training at the time
Squatting again starting with 70kg for about 20 reps
110kg for 15, then go to around 140kg were I would start to get serious, increasing the weight and drop reps up to around 180-200kg
If though for example I was to do flyes straight after bench, I would normally use around 25-27kg dumbells (depending on how tight my chest and shoulders are feeling) before moving up to 32-35kg
Warming up with lighter moderate weights for higher reps, pumps blood into the muscle area you are about to work, loosens the joints up and also helps get the mind set on what you are about to do.
There is absolutely no way the same could be achieved from using a heavy weight and doing the movement slow. Every movement should be reasonably slow and controlled anyway.
