I only tow the dinghy when I am not travelling but make short jumps on the cruising ground (less than 30nm) and with time and experience I have come to some conclusions:
1-never tow the dinghy with the engine or oars inside. It makes it a lot slower and in the eventual situation of strong winds you can get a capsized dinghy and lost oars or damage the engine (happened several times including this year, I mean capsizing the dinghy).
2- pull it really close to the boat and use some system that can provide a lift of the dinghy bow. That will give significant less drag.
I don't like to use the back-stay for that and use a system that provides an attenuation of efforts (kind of suspension) and the distribution of efforts by several parts of the boat. Main effort (horizontal pull) is on some strong part of the boat, the lifting pull is on the back life lines (not much force).
Regarding the dinghy I distribute efforts by the two lateral D rings and normally pass a security line on the one in the middle, just in case.
Because the bridle (bowline knot) is big most of the force is forward and as the dingy is keep near the boat and pulled up, even with waves, the lateral pulls are very rare and not strong.
With this system you only lose about half a knot speed, about the same you lose with a fixed propeller to a feather one.