This is my home network:
Laptop connected to BOSS
When my laptop is connected directly via WiFi to the main modem-router (SSID BOSS), the speed is 76 Mbps.
Laptop connected to TORCHWOOD
When my laptop is connected via WiFi to PLC-WIFI (SSID TORCHWOOD), the speed between the TP Link PLCs is 17 Mbps
I do not understand the cause of this speed loss.
That sounds about right. Homeplug is usually slower than the rated speeds, and is affected by multiple things - line quality, the presence and distance between your homeplug devices and 'noisy' devices. Our dryer used to cause my network connection to go down, for example. I have an answer here that goes into that in somewhat more detail. Essentially any network segment is only as good as its weakest link.
Much like wifi, a good idea is to experiment and work out the best locations for such plugs and work out what the slowdowns are . Try the homeplug units at a nearer plug for example.
TPlink should have a tool for predicting speeds, which while apparently inaccurate, might give you a relative idea of signal quality.
While my current setup dosen't support it, homeplug monitor is a much better tool for powerusers, and lets you see SNR and such.
I'd finish by saying, your wifi speed and powerline networking speed are independant of each other and have different criteria affecting performance.
I don't know the layout of your house, but it is likely that there is either a physical barrier (solid wall or floor you may not know about?) in between to of the wirelessly connected components, or there may be some sort of electromagnetic interference from another device (plasma TVs cause a lot of problems). I would check for anything that may be causing interference and move your relay (SSID TORCHWOOD) away from them. If that does not solve your problem, the delay is likely between your router and your signal relay (SSID TORCHWOOD)
