Networking – Wired homeplug is ridiculously slow

I've had a mixed homeplug network consisting of a 500mbps/4 port gig-e switch and a pair of 200 mbps non passthrough adaptors. Its reasonably similar to yours I've found a few things troubleshooting it. You can see a somewhat stripped down network diagram in my answer here

Firstly that not all adaptors in the network behave identically - the gig-e switch is significantly more sensitive to noise (though I have no idea to whether this is due to it being in a mixed network, or inherently sensitive to noise). While useless in most cases if the signal lights report worse than perfect signal, you're probably alright on the power line side of things. Oddly I found its more likely something is wrong with your devices report a perfect signal, but you have connection issues.

As for noise, the only real way to work this out is to go on walkabout - my switch goes down whenever one of our washing machines run (the 200 mbps gear has signal degradation). The troubleshooting methods available on homeplug gear is rather useless - in my case the switch reports perfect signal (Its usually red or amber) when it isn't working, and occationally blinks. I started by switching off all other devices in my room, ruled out any changes in the next room and the next when the networking was glitching, and worked my way through the apartment. Focusing on devices with big motors like Fridges and washing machines is a good idea. You can probably add a filter, but I haven't found one for my local plug type. Some people actually use passthrough homeplugs for the same thing, but it seems wasteful. I ended up fixing this issue by moving the homeplug connection further from the devices that gave me issues, and I have had no issues since.

I'd also 'check' to make sure all the homeplugs are on the same private network name - they generally tend to be set to HomeplugAV, but changing it would be a good idea. You can set it with magic everything button Homeplug units have, or use a utility provided by the company that makes them (which tend to not actually tell you what the current network name is, or actually provide any sort of feedback whatsoever). In theory you can also detect what devices are on the network, but practically this works rarely enough (even when I'm on the network, and connected properly) that its pointless.

In your case it was something else, but I'd like to put this out there since anyone looking for my problem or something similar

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.