Sometimes loss of swing speed with the driver occurs when our swing becomes a little narrow. In the take away try to keep the clubhead close to the ground for as long as possible , this will give you plenty of width on the backswing and should improve your clubhead speed . If this dosn't improve your clubhead speed you could be casting (releasing the club too early ) slightly on your down swing . try retain that 45degree angle you achieve at the top of your backswing for a little longer on the downswing ( you should hear a nice audible SWISH through impact) ........good luck