Power loss? It's direct shaft. Although I don't know of a way to measure brake horsepower, there is no transmission loss as I understand because it's direct shaft, but I may be wrong.
There are plenty of guys getting 20 mph with longtails like this from simple 6.5 hp powerplants like this: because it uses a go-kart powerplant you can hot-rod it just like you would a small-block 350. Considering I can build one of these for less than $600, then modify it to produce twice as much horsepower, it's not a bad deal.
There's also plenty of guys running V-twin 22 hp longtails running 25 mph. That's more speed with less horsepower: