Why do my P.L.Cs (Power line converters) devide the speed by two?

If I directly connect to my 100MB router with a CAT.5e cable, I get these results :

$ speedtest --simple 
Ping: 8.617 ms
Download: 92.54 Mbit/s
Upload: 93.90 Mbit/s
$ ethtool eth0
Settings for eth0:
    Supported ports: [ TP MII ]
    Supported link modes:   10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
    Supported pause frame use: No
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
    Advertised pause frame use: Symmetric Receive-only
    Advertised auto-negotiation: Yes
    Link partner advertised link modes:  10baseT/Half 10baseT/Full 
                                         100baseT/Half 100baseT/Full 
    Link partner advertised pause frame use: Symmetric Receive-only
    Link partner advertised auto-negotiation: Yes
    Speed: 100Mb/s
    Duplex: Full
    Port: MII
    PHYAD: 0
    Transceiver: internal
    Auto-negotiation: on
    Current message level: 0x00000033 (51)
                   drv probe ifdown ifup
    Link detected: yes

But if I connect to my same router through two dLAN® 200 AVmini PLCs (one power line converter connected to the router and the other to my PC), I get this :

$ speedtest --simple
Ping: 12.843 ms
Download: 43.86 Mbit/s
Upload: 49.06 Mbit/s
$ ethtool eth0
Settings for eth0:
    Supported ports: [ TP MII ]
    Supported link modes:   10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
    Supported pause frame use: No
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
    Advertised pause frame use: Symmetric Receive-only
    Advertised auto-negotiation: Yes
    Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                         100baseT/Half 100baseT/Full
    Link partner advertised pause frame use: No
    Link partner advertised auto-negotiation: Yes
    Speed: 100Mb/s
    Duplex: Full
    Port: MII
    PHYAD: 0
    Transceiver: internal
    Auto-negotiation: on
    Current message level: 0x00000033 (51)
                   drv probe ifdown ifup
    Link detected: yes 

The difference between the two is :

Ping: 8.617 ms                                                     |    Ping: 12.843 ms
Download: 92.54 Mbit/s                                             |    Download: 43.86 Mbit/s
Upload: 93.90 Mbit/s                                               |    Upload: 49.06 Mbit/s
    Link partner advertised pause frame use: Symmetric Receive-only|        Link partner advertised pause frame use: No

Why is the speed divided by two and how can I solve this ?

EDIT0 : One can read the dLAN® 200 AVmini product sheet and HomePlug wikipedia article.

Your powerline converters are built to an old standard (HomePlug AV, from 2005) that doesn't provide very good performance. 50Mbps is about what you get under realistic conditions with this class of converter.

I would strongly urge you to upgrade to converters built to the newer AV2 standard from 2012. Devolo's "Magic 1 LAN" is built to the newer standard and should provide significantly better performance.

You can try plugging your converters into different outlets. Do not plug them into extension cords or power strips. You might find a configuration change that gets you some improvement.