Do the speed ratings differ between winter and summer tyres?
Yes. In general winter tyres have a lower speed rating than summer tyres. This is because of the softer nature of winter tyres, the benefit of which is increased grip in icy or snowy conditions.
However, winter tyres shouldn't compromise your driving style as even H rated tyres carry a maximum speed of 131mph, more than enought for most of us. You should however take more care when driving during darker winters with more hazardous conditions.