Slow Internet speed when HomePlug combined with router
What happens if you plug both powerline devices into the same plug strip AND turn off every circuit breaker in the house except the one powering this circuit? (i.e. Eliminate all possible sources of powerline noise.) If you find this resolves your speed issue then turn breakers back on one at a time until you find which has the noise source (and it may not just be one, but you get the idea). The difference between protocols I expect is exactly that... other protocols have better error correction than TCP.