Below the hull speeds, in light air, it's a different issue than in moderate or stronger winds.

I've not towed or ever had a hard dingy mystelf. I've found that a simple inflatable (WITHOUT LOADING of water, engine, fuel, gear, etc.) generally can be adjusted to it's load is pretty minimal. If you're in light winds, towing a line can affect speed through the water ;^))) In modest winds (say 8 to 10 knots), I don't think it's a 1/2 knot. Above 10 knots, I think it should be less. However, put 10 gallongs of water, 5 gallons of fuel, and an outboard on the back -- and you have a different story ;^)))